Improve mingw support.

- Remove existing mingw-w64 toolchain file.
- Add x86 and x86_64 mingw-w64 toolchain files.
- Add gtest mingw work around (disable gtest cmake and build
  our own static lib).

Change-Id: Ic602df62f5485ccbd79deccf3aed2a4d35fef8ad
4 files changed