First, use original cortex-a15 substitution, if no target_clang_version, else,
if msm-3.4 clang and krait TARGET_VARIANT_CPU, use optimized krait2
for the only 2 posibilities in that situation to be affcted:
cortex-a15 and cortex-a9

Change-Id: I1e8e57ac4a36465200e19e2293a75305edc0c2fe