build: clang: use swift flags for krait, denver, and A15

Move definition of -D__ARM_FEATURE_LPAE=1 cflag to top-level.

LPAE indicates better instructions can be used when atomicity guarantees are
needed. However, LPAE's presence isn't advertised by clang/GCC. We fake an
ARM feature to advertise its presence on architectures where it is.
Also, add a TODO documenting that cortex-a15 is not the correct CPU variant
for krait.

(From Ian Rogers at AOSP)

Change-Id: I02a1248025c32d94eca0bc8a249dc524f1ac9c36