Add --minimal option to bootstrap
Add --minimal option to bootstrap

Currently reduces LLVM build:

- no docs
- only host,x86_64,ptx,arm targets

This oddly fails to build due to build system errors on OS X, but seems
to work on Ubuntu. This is an LLVM bug. (It thinks it libLTO.dylib
depends on the Hexagon backend, even when that target is disabled.)


Former-commit-id: 182d0247155420ca86dbd99d7f4c925daca09ed9
darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide