cmake: gdb: use gdb-multiarch if ${CROSS_COMPILE}gdb is not availiable

Modern versions of GDB support multiple architectures
with the same binary.
In fact, Ubuntu stopped shipping a gdb-arm-none-eabi
package, gdb-multiarch should be used instead.

This fixes a failure on those systems where otherwise CMAKE_GDB will be
assigned to CMAKE_GDB-NOTFOUND.

Signed-off-by: Benjamin Valentin <benjamin.valentin@ml-pa.com>