Update build.py to support Windows

Bug: http://b/28196329

This change adds CMake configuration to cross-compile Clang for Windows
using MinGW.  This also refactors LLVM build steps to a separate
function to enable reuse.

Test: Run llvm/android/build.py on Linux and Darwin

Change-Id: Ibfa9db4d8030b921c3fe874f9208e084283f5f7f
1 file changed