Make libwinpthread a prebuilt_cc_library_static am: a6fee370be

Original change: https://android-review.googlesource.com/c/platform/prebuilts/gcc/linux-x86/host/x86_64-w64-mingw32-4.8/+/1950276

Change-Id: Ie6d81a742c64164df1e52026f6a256bac7b46103