makefiles/tools: Automatically detect gdb-multiarch
makefiles/tools: Automatically detect gdb-multiarch

Modern versions of GDB support multiple targets with the same gdb binary.
At least Ubuntu and Debian have dropped the gdb-arm-none-eabi package in favour
of gdb-multiarch.
Here, no $(PREFIX)-gdb binary is availiable, instead gdb-multiarch should be used.

This patch tries to automatically detect the presense of gdb-multiarch and uses it
instead of arm-none-eabi-gdb.

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