Switch to hermetic GCC 4.8.3-based x86_64-w64-mingw32-4.8 compiler.

Bug: 14416410

prebuilts/gcc/linux-x86/host/x86_64-w64-mingw32-4.8/x86_64-w64-mingw32
is the location for the checked-in hermetic mingw Windows compiler. This
removes our reliance on host mingw when building with "make win_sdk".

Change-Id: I1f9bbb85912d9855be32f9cab4bdfd12d6d3246c