build: allow to customize the kernel toolchain cpu variant

- for devices with cortex-a7 like the moto g, this way they can still take advantage of neon-vfp4 and other things

Change-Id: I4b63570d21affeba7c20da6aadc6cefda4293eb2
Merge "build: add support for a prebuilt clang" into kk44
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.

http://pastebin.com/FaxSUzVy

ps5: don't enable it by default because of poor device support

Signed-off-by: bart452 <bart_96@live.nl>

Change-Id: I17ea1d46cf1ba9bace70acd89a80143060f3bc31