Merge changes from topic "build-ndk-crt"

* changes:
  Reapply: Hack libc++ support header to work in the platform.
  Update NDK prebuilts to build 6528147.
  Update NDK prebuilt update script.
  Stop exposing prebuilt CRT objects to soong.
tree: bcf3c9ae6ea7569e1c2c6ebed9d8de4c8f93470e
  1. r21/
  2. Android.bp
  3. Android.mk
  4. CleanSpec.mk
  5. NOTICE
  6. pylintrc
  7. update.py