xcompile: always use -march=i686

When compiling coreboot for x86 on gcc the compiler is
free to pick whatever defaults it is using at the time of
gcc's compile/configuration when no -march is specified.
Not properly specifying -march then opens up the use of SSE
instructions for copmilation units it should not be used such
as the SMM module as this module doesn't save/restore SSE
registers.

BUG=chrome-os-partner:22991
BRANCH=None
TEST=Built and confirmed -march=i686 was used on command line. Also
     noted not xmm registers were produced grep'ing through objdump
     output.

Change-Id: I64d4a6c5fa9fadb4b35bc7097458e992a094dcba
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/172640
Reviewed-by: Stefan Reinauer <reinauer@google.com>

util/xcompile/xcompile

@@ -106,6 +106,11 @@ detect_special_flags() {
 		# Use bfd linker instead of gold if available:
 		testcc "$CC"   "$CFLAGS -fuse-ld=bfd" &&
 			CFLAGS="$CFLAGS -fuse-ld=bfd" && LINKER_SUFFIX_i386='.bfd'
+		# Always build for i686 -- no sse/mmx instructions since SMM
+		# modules are compiled using these flags. Note that this
+		# doesn't prevent a project using xcompile to explicitly
+		# specify -mmsse, etc flags.
+		CFLAGS="$CFLAGS  -march=i686"
 		;;
 	arm )
 		ARMFLAGS=""