Fix arm[64] cflags

Limit arm cpu variants to armv7-a-neon only, and handle a generic
armv7-a-neon cpu variant. Add arm64 cortex-a53 cpu variant. Sanity check
the arch variant against supported versions.

Change-Id: I8ecdea4cd36d9c82e1f29b158efc131f8b1cbc4d
4 files changed