cpu/cortexm_common: call hard_fault_handler using bl
cpu/cortexm_common: call hard_fault_handler using bl

Fixes this error when building with Clang 3.7.1:

fatal error: error in backend: unsupported relocation on symbol
x86_64-pc-linux-gnu-clang-3.7: error: clang frontend command failed with exit code 70 (use -v to see invocation)
clang version 3.7.1 (tags/RELEASE_371/final)
Target: arm-none--eabi
Thread model: posix
x86_64-pc-linux-gnu-clang-3.7: note: diagnostic msg: PLEASE submit a bug report to http://llvm.org/bugs/ and include the crash backtrace, preprocessed source, and associated run script.
x86_64-pc-linux-gnu-clang-3.7: note: diagnostic msg:
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
x86_64-pc-linux-gnu-clang-3.7: note: diagnostic msg: /tmp/vectors_cortexm-54333e.c
x86_64-pc-linux-gnu-clang-3.7: note: diagnostic msg: /tmp/vectors_cortexm-54333e.sh
x86_64-pc-linux-gnu-clang-3.7: note: diagnostic msg:

********************

BytesGalore/RIOT,katezilla/RIOT,beurdouche/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,wentaoshang/RIOT,biboc/RIOT,d00616/RIOT,dailab/RIOT,TobiasFredersdorf/RIOT,rfuentess/RIOT,avmelnikoff/RIOT,kaleb-himes/RIOT,altairpearl/RIOT,OlegHahm/RIOT,FrancescoErmini/RIOT,toonst/RIOT,shady33/RIOT,LudwigOrtmann/RIOT,lazytech-org/RIOT,zhuoshuguo/RIOT,MohmadAyman/RIOT,LudwigOrtmann/RIOT,mfrey/RIOT,rousselk/RIOT,FrancescoErmini/RIOT,OTAkeys/RIOT,backenklee/RIOT,mtausig/RIOT,mfrey/RIOT,katezilla/RIOT,lazytech-org/RIOT,alignan/RIOT,jremmert-phytec-iot/RIOT,cladmi/RIOT,wentaoshang/RIOT,syin2/RIOT,cladmi/RIOT,malosek/RIOT,dkm/RIOT,d00616/RIOT,biboc/RIOT,malosek/RIOT,rousselk/RIOT,kaleb-himes/RIOT,alignan/RIOT,aeneby/RIOT,A-Paul/RIOT,rakendrathapa/RIOT,wentaoshang/RIOT,roberthartung/RIOT,roberthartung/RIOT,malosek/RIOT,ks156/RIOT,kYc0o/RIOT,tfar/RIOT,x3ro/RIOT,RIOT-OS/RIOT,smlng/RIOT,neumodisch/RIOT,zhuoshuguo/RIOT,malosek/RIOT,A-Paul/RIOT,yogo1212/RIOT,kYc0o/RIOT,basilfx/RIOT,zhuoshuguo/RIOT,rakendrathapa/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,neumodisch/RIOT,jfischer-phytec-iot/RIOT,ks156/RIOT,cladmi/RIOT,hamilton-mote/RIOT-OS,rajma996/RIOT,hamilton-mote/RIOT-OS,neumodisch/RIOT,rfuentess/RIOT,jasonatran/RIOT,josephnoir/RIOT,gebart/RIOT,jremmert-phytec-iot/RIOT,tfar/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,TobiasFredersdorf/RIOT,Hyungsin/RIOT-OS,josephnoir/RIOT,rajma996/RIOT,plushvoxel/RIOT,BytesGalore/RIOT,adjih/RIOT,syin2/RIOT,lazytech-org/RIOT,alignan/RIOT,backenklee/RIOT,kerneltask/RIOT,attdona/RIOT,rfuentess/RIOT,authmillenon/RIOT,kYc0o/RIOT,MohmadAyman/RIOT,MohmadAyman/RIOT,syin2/RIOT,d00616/RIOT,immesys/RiSyn,immesys/RiSyn,kaspar030/RIOT,jfischer-phytec-iot/RIOT,yogo1212/RIOT,beurdouche/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,ant9000/RIOT,neumodisch/RIOT,RIOT-OS/RIOT,basilfx/RIOT,kaspar030/RIOT,mtausig/RIOT,altairpearl/RIOT,LudwigKnuepfer/RIOT,adjih/RIOT,BytesGalore/RIOT,lazytech-org/RIOT,OlegHahm/RIOT,rousselk/RIOT,kaspar030/RIOT,asanka-code/RIOT,backenklee/RIOT,TobiasFredersdorf/RIOT,kb2ma/RIOT,toonst/RIOT,LudwigOrtmann/RIOT,FrancescoErmini/RIOT,kaspar030/RIOT,zhuoshuguo/RIOT,plushvoxel/RIOT,kbumsik/RIOT,Yonezawa-T2/RIOT,gebart/RIOT,toonst/RIOT,brettswann/RIOT,basilfx/RIOT,rajma996/RIOT,thomaseichinger/RIOT,immesys/RiSyn,adrianghc/RIOT,Yonezawa-T2/RIOT,jasonatran/RIOT,shady33/RIOT,lazytech-org/RIOT,LudwigKnuepfer/RIOT,attdona/RIOT,jbeyerstedt/RIOT-OTA-update,wentaoshang/RIOT,Josar/RIOT,jremmert-phytec-iot/RIOT,A-Paul/RIOT,A-Paul/RIOT,miri64/RIOT,ant9000/RIOT,jremmert-phytec-iot/RIOT,jasonatran/RIOT,gautric/RIOT,BytesGalore/RIOT,smlng/RIOT,OlegHahm/RIOT,gebart/RIOT,dkm/RIOT,A-Paul/RIOT,beurdouche/RIOT,Yonezawa-T2/RIOT,toonst/RIOT,authmillenon/RIOT,immesys/RiSyn,adjih/RIOT,rousselk/RIOT,shady33/RIOT,Ell-i/RIOT,plushvoxel/RIOT,malosek/RIOT,mfrey/RIOT,rakendrathapa/RIOT,MohmadAyman/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,kerneltask/RIOT,josephnoir/RIOT,kb2ma/RIOT,OTAkeys/RIOT,backenklee/RIOT,roberthartung/RIOT,neumodisch/RIOT,biboc/RIOT,smlng/RIOT,shady33/RIOT,mtausig/RIOT,hamilton-mote/RIOT-OS,rakendrathapa/RIOT,mtausig/RIOT,malosek/RIOT,mtausig/RIOT,thomaseichinger/RIOT,Hyungsin/RIOT-OS,cladmi/RIOT,LudwigKnuepfer/RIOT,gautric/RIOT,tfar/RIOT,jasonatran/RIOT,kYc0o/RIOT,dailab/RIOT,TobiasFredersdorf/RIOT,asanka-code/RIOT,ks156/RIOT,jbeyerstedt/RIOT-OTA-update,plushvoxel/RIOT,TobiasFredersdorf/RIOT,shady33/RIOT,backenklee/RIOT,gautric/RIOT,Ell-i/RIOT,tfar/RIOT,OTAkeys/RIOT,adrianghc/RIOT,basilfx/RIOT,kbumsik/RIOT,smlng/RIOT,dkm/RIOT,jfischer-phytec-iot/RIOT,aeneby/RIOT,d00616/RIOT,kbumsik/RIOT,hamilton-mote/RIOT-OS,dailab/RIOT,dkm/RIOT,beurdouche/RIOT,attdona/RIOT,kerneltask/RIOT,attdona/RIOT,Ell-i/RIOT,LudwigOrtmann/RIOT,dailab/RIOT,rousselk/RIOT,tfar/RIOT,miri64/RIOT,LudwigOrtmann/RIOT,jbeyerstedt/RIOT-OTA-update,kbumsik/RIOT,kerneltask/RIOT,rajma996/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,neiljay/RIOT,d00616/RIOT,asanka-code/RIOT,rousselk/RIOT,katezilla/RIOT,MohmadAyman/RIOT,authmillenon/RIOT,jfischer-phytec-iot/RIOT,jremmert-phytec-iot/RIOT,x3ro/RIOT,Hyungsin/RIOT-OS,wentaoshang/RIOT,josephnoir/RIOT,kbumsik/RIOT,attdona/RIOT,FrancescoErmini/RIOT,brettswann/RIOT,x3ro/RIOT,Josar/RIOT,plushvoxel/RIOT,kerneltask/RIOT,brettswann/RIOT,kaspar030/RIOT,altairpearl/RIOT,mfrey/RIOT,yogo1212/RIOT,yogo1212/RIOT,ant9000/RIOT,ks156/RIOT,Yonezawa-T2/RIOT,dailab/RIOT,brettswann/RIOT,Ell-i/RIOT,avmelnikoff/RIOT,yogo1212/RIOT,smlng/RIOT,altairpearl/RIOT,kb2ma/RIOT,adjih/RIOT,rakendrathapa/RIOT,avmelnikoff/RIOT,adrianghc/RIOT,x3ro/RIOT,brettswann/RIOT,immesys/RiSyn,Josar/RIOT,asanka-code/RIOT,OlegHahm/RIOT,jbeyerstedt/RIOT-OTA-update,d00616/RIOT,ks156/RIOT,roberthartung/RIOT,kb2ma/RIOT,asanka-code/RIOT,thomaseichinger/RIOT,syin2/RIOT,FrancescoErmini/RIOT,biboc/RIOT,aeneby/RIOT,biboc/RIOT,authmillenon/RIOT,asanka-code/RIOT,immesys/RiSyn,altairpearl/RIOT,katezilla/RIOT,avmelnikoff/RIOT,Yonezawa-T2/RIOT,yogo1212/RIOT,kb2ma/RIOT,OlegHahm/RIOT,aeneby/RIOT,Ell-i/RIOT,jasonatran/RIOT,BytesGalore/RIOT,basilfx/RIOT,neiljay/RIOT,aeneby/RI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