cmake: bintools: gnu: Do not use gdb-multiarch

The commit c2419fc0 introduced a change
to the CMake toolchain detection routine in which it attempts to use
the distro-provided `gdb-multiarch` GDB executable when the target-
specific GDB executable cannot be found (i.e. fallback to
`gdb-multiarch`).

This can cause many problems for the Zephyr users because the distro-
provided GDB does not contain any Zephyr-specific patches and it may
misbehave under various conditions (e.g. negative tid/pid being
displayed, Xtensa debugging mysteriously failing, ARC debugging not
working).

This commit removes the fallback mechanism to the distro-provided
`gdb-multiarch` because it is better to display an error message saying
GDB is not found than to use a wrong GDB that can give out very
confusing and misleading error messages to users.

Moreover, this fallback mechanism is no longer necessary because the
Zephyr SDK is now available on all major host operating systems and it
provides the target-specific custom GDB that contains all Zephyr-
specific patches.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>