Fix arm cflags

-mthumb-interwork was missing from the beginning.

The Clang cflags copy was only a shallow copy, so our clang-specific
fixups were affecting the GCC cflags as well.

The cortex-a15 replacement was removed from build/core with this change:
https://android-review.googlesource.com/#/c/143851

Change-Id: Ib6300e064f6ec4f6a7174d5e2aeacad1c359f7ea
1 file changed