Modify the method to patch synthmark

The old way to patch synthmark kept a .mk file which will cause build
failing. Have put the .mk file patch into a diff file so that Android
building system will not see a .mk file here.

BUG=None
TEST=Checkbuild succeeded with this patched to external/toolchain-utils
on Android side.

Change-Id: I704aa02d9ca91098727786f053aeb26e989bd481
Reviewed-on: https://chromium-review.googlesource.com/581869
Commit-Ready: Zhizhou Yang <zhizhouy@chromium.org>
Tested-by: Zhizhou Yang <zhizhouy@chromium.org>
Reviewed-by: George Burgess <gbiv@chromium.org>
4 files changed