Add support for detecting an x86-64 cross compiler in xcompile. Change-Id: Icd2c9af7903956216db1fd54902eab6da0fe3e21 Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org> Signed-off-by: Scott Duplichan <scott@notabs.org> Reviewed-on: http://review.coreboot.org/8669 Tested-by: build bot (Jenkins) Reviewed-by: Patrick Georgi <pgeorgi@google.com>