subst mtune=cortex-a15 to krait2 for msm-% clang device...
and march=armv7-a for prebuilt

Change-Id: I6206057e0796ba4db46be4c0bf1a04ea3ade6ad0
New changes needed for msm-3.5 clang

Change-Id: If41682c7265645056be757e58eb14b8707ce7438