xcompile: ask for compiler runtime using appropriate CFLAGS

xcompile keeps two CFLAGS around now, for GCC and CLANG. Normally they're not
required to request the libgcc/compiler-rt path, but with the multilib capable
x86_64-elf target it's required to make it pick the right libgcc when used as
i386-elf builder.

Change-Id: I700e7aa5783dc36698dd2ab8a38642a144e80fe9
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/10795
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins)
1 file changed