Unify HAVE_GDB_BIN (currently in config-host.mak) and
HOST_GDB_SUPPORTS_ARCH into a single GDB variable in
config-target.mak.

Reviewed-by: Emmanouil Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>