[libunwind] Treat assembly files as C on mingw

When targeting mingw, current CMake (3.16) fails to get the right
flags for assembly source files for windows gnu/clang targets
(see https://gitlab.kitware.com/cmake/cmake/merge_requests/4287
for a fix), causing builds to fail due to `-fPIC` being unsupported
in clang for mingw targets

In the meantime, restore the behaviour from before c48974ff
selectively on mingw targets, treating the assembly files as C.

Differential Revision: https://reviews.llvm.org/D73436