fix GCC 4.9 compilation errors
Revert "Revert "ARM: Use -mcpu=cortex-a15 when targeting MSM Krait CPUs""

This reverts commit 23e168f601ba0085bb6dc549f00cdee22c56bdbe.
Fix build.

We already building kernel as cortex-a15

Change-Id: I9764fa8d2e38d3c3d456b6ee3d2743b036ba1889