build.py: Implement "gcc" builder
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.
Subscribers: mehdi_amini, dexonsmith, lldb-commits
Differential Revision: https://reviews.llvm.org/D55430