cortexm_common: don't try to set MEMFAULTENA on ARMv6-M
cortexm_common: don't try to set MEMFAULTENA on ARMv6-M

Before this change, if one tried to build a Cortex-M0+ target that had
an MPU, compilation would fail due to missing
'SCB_SHCSR_MEMFAULTENA_Msk' in SCB structure. Cortex-M0+ is a ARMv6-M
arch (unlike most other targets that have MPU support). ARMv6-M has more
limited support for fault conditions, see ARMv6-M Architecture Reference
Manual, D3.6.2.

mtausig/RIOT,kYc0o/RIOT,miri64/RIOT,authmillenon/RIOT,aeneby/RIOT,kaspar030/RIOT,kYc0o/RIOT,neiljay/RIOT,OlegHahm/RIOT,avmelnikoff/RIOT,kbumsik/RIOT,mtausig/RIOT,miri64/RIOT,lazytech-org/RIOT,A-Paul/RIOT,authmillenon/RIOT,A-Paul/RIOT,kbumsik/RIOT,OTAkeys/RIOT,neiljay/RIOT,toonst/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,neiljay/RIOT,gebart/RIOT,rfuentess/RIOT,smlng/RIOT,jasonatran/RIOT,toonst/RIOT,mtausig/RIOT,biboc/RIOT,BytesGalore/RIOT,rfuentess/RIOT,kaspar030/RIOT,BytesGalore/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,smlng/RIOT,authmillenon/RIOT,kaspar030/RIOT,aeneby/RIOT,mfrey/RIOT,lazytech-org/RIOT,A-Paul/RIOT,smlng/RIOT,toonst/RIOT,gebart/RIOT,kYc0o/RIOT,x3ro/RIOT,A-Paul/RIOT,OTAkeys/RIOT,kbumsik/RIOT,x3ro/RIOT,lazytech-org/RIOT,biboc/RIOT,yogo1212/RIOT,OlegHahm/RIOT,kbumsik/RIOT,avmelnikoff/RIOT,toonst/RIOT,OlegHahm/RIOT,lazytech-org/RIOT,mfrey/RIOT,cladmi/RIOT,x3ro/RIOT,basilfx/RIOT,kbumsik/RIOT,x3ro/RIOT,lazytech-org/RIOT,mfrey/RIOT,mtausig/RIOT,RIOT-OS/RIOT,gebart/RIOT,yogo1212/RIOT,mfrey/RIOT,josephnoir/RIOT,avmelnikoff/RIOT,josephnoir/RIOT,aeneby/RIOT,ant9000/RIOT,mtausig/RIOT,miri64/RIOT,authmillenon/RIOT,miri64/RIOT,ant9000/RIOT,aeneby/RIOT,yogo1212/RIOT,smlng/RIOT,mfrey/RIOT,kYc0o/RIOT,basilfx/RIOT,rfuentess/RIOT,josephnoir/RIOT,BytesGalore/RIOT,yogo1212/RIOT,smlng/RIOT,kYc0o/RIOT,jasonatran/RIOT,basilfx/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,rfuentess/RIOT,biboc/RIOT,kaspar030/RIOT,cladmi/RIOT,ant9000/RIOT,avmelnikoff/RIOT,kaspar030/RIOT,ant9000/RIOT,OTAkeys/RIOT,neiljay/RIOT,basilfx/RIOT,biboc/RIOT,ant9000/RIOT,authmillenon/RIOT,x3ro/RIOT,gebart/RIOT,yogo1212/RIOT,basilfx/RIOT,miri64/RIOT,neiljay/RIOT,josephnoir/RIOT,rfuentess/RIOT,cladmi/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,biboc/RIOT,jasonatran/RIOT,OlegHahm/RIOT,BytesGalore/RIOT,josephnoir/RIOT,cladmi/RIOT,jasonatran/RIOT,cladmi/RIOT,avmelnikoff/RIOT,BytesGalore/RIOT,aeneby/RIOT,gebart/RIOT,toonst/RIOT,yogo1212/RIOT,A-Paul/RIOT