[lanai] Add Lanai backend to clang driver.

Changes to clang to add Lanai backend. Adds a new target, ABI and toolchain.

General Lanai backend discussion on llvm-dev thread "[RFC] Lanai backend" (http://lists.llvm.org/pipermail/llvm-dev/2016-February/095118.html).

Differential Revision: http://reviews.llvm.org/D17002