Use -mcpu=cortex-a15 for krait

Based on https://android.googlesource.com/platform/build/+/d195c6d14e027f12f63508afed245a1ce469c936

Change-Id: Ib409c915c5302cdb44d49978456b7d1cc6a2a250
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>