core/clang: improve clang configuration & aarch64 support:

Summary:
We can get away with this configuration due to supporting all
targets in our toolchain prebuilts. Squashed of the following:
  - Introduce TARGET_CFX_CLANG_VERSION and TARGET_CFX_CLANG_VERSION_EXP
  - Introduce proper aarch64 support (we have support in our prebuilts)

Test Plan: win

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D41