xcompile: ABIs are really architecture specific

no need to test for i386-eabi or armv7a-elf

Change-Id: Icbef5a64f5b793092ca0f94ee8f54bc896bf39ad
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/5746
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>