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 compilation units it should not be used such
as the SMM module as this module doesn't save/restore SSE

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>
(cherry picked from commit d49358f7959bb52c3e7ff67d37c21a1b294adf72)
Signed-off-by: Isaac Christensen <isaac.christensen@se-eng.com>
Reviewed-on: http://review.coreboot.org/6716
Tested-by: build bot (Jenkins)
1 file changed