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.
6e7414e220