buildgcc: Fix wrapper Makefile

buildgcc: Fix wrapper Makefile

buildgcc moved from building gdb by default (with opt-out) to
gdb being optional. Adapt Makefile so it works again

Change-Id: I663a8c70db4f7b5d07456fb67a223dbb2de2c133
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/417
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>