Introduce Link Time Optimization

Summary:
This introduces the following improvements:
  - Can be globally enabled with TARGET_ENABLE_LTO := true
  - Can be locally disabled with LOCAL_NO_LTO_SUPPORT.
  - Host modules now build without LTO by default.
  - Modules built with clang no longer need -fno-lto specified locally (LTO disabled here automatically)
  - Modules built with clang no longer have -fno-toplevel-reorder unnecessarily set.

Change-Id: Idc13bc51df4711dab2dc7a3f7214618d34622dc7