[gn build] Add build files for //bolt

Adds build files for libraries, tools, and tests -- everything except
the runtime.

Doesn't hook up bolt in the main BUILD.gn file yet -- I want to verify
that it builds on Linux, macOS, Windows before doing that. (I've only
checked on macOS so far.)

`ninja check-bolt` passes on macOS with this.
(I locally bumped the deployment target to macOS 10.12 for that. bolt/ uses
std::mutex quite a bit, which requires 10.12.)

Differential Revision: https://reviews.llvm.org/D138355