Update xcompile to search for x86_64 toolchain.

This adds detection of x86_64 gcc toolchain (which buildgcc can build
if provided the option).

Change-Id: I8b12f3e705157741279c7347f4847fb50ccc2b0e
Signed-off-by: Marc Jones <marc.jones@se-eng.com>
Reviewed-on: http://review.coreboot.org/673
Reviewed-by: Rudolf Marek <r.marek@assembler.cz>
Tested-by: build bot (Jenkins)