build.py: Implement "gcc" builder

Summary:
This implements the gcc builder in build.py script to allow it to
compile host executables when running on a non-windows host. Where it
made sense, I tried to share code with the msvc builder by moving stuff
to the base class.

Reviewers: zturner

Subscribers: mehdi_amini, dexonsmith, lldb-commits

Differential Revision: https://reviews.llvm.org/D55430