Fix prebuilts for target builds with USE_MINGW=1.

USE_MINGW=1 mm didn't work in directories that contained target modules
because the build system would use the Windows locations and extensions
when trying to find the host GCC prebuilts. Windows is the target OS,
not the OS we're building from.

Change-Id: Ic994fed15388d0c7d393f71ba28fe7afdc659f5c