Fix building for mingw/armv7 with Clang 13
Pass the -mimplicit-it option through with -Wa.
GCC never supported this option as a compiler option, it's only a gas option, that should be passed through with -Wa. Clang used to accept it as a compiler option, but Clang 13 became stricter and no longer accepts it at that level, only passed through with -Wa.