util/xcompile/xcompile: apply -march to clang as well as gcc

util/xcompile/xcompile: apply -march to clang as well as gcc

For x64 and x86_32 configurations, apply the -march flag to both GCC and
Clang flags.

This solves the problem of Clang-compiled coreboot failing due to Clang
emitting SSE instructions for code that is executed while SSE is not
enabled.

This patch takes functionality targeted for GCC configurations and moves
it down a few lines, modifying CFLAGS instead of GCC_CFLAGS in order
that it applies to both GCC and Clang.

This is an alternate patch to CB:32887.

Signed-off-by: Alan Green <avg@google.com>
Change-Id: I6a6a6136b01a64d46f730ed19ebbeaadaf2183df
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32923
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>