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

TEST=Built and confirmed -march=i686 was used on command line. Also
     noted not xmm registers were produced grep'ing through objdump

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>
1 file changed