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.
13 jobs for implicit-it in 4 minutes and 3 seconds (queued for 3 seconds)
Name Stage Failure
build-llvm-mingw-armv7 Build
make: *** [Makefile:306: common/arm/mc-a-8.o] Broken pipe
make: *** [Makefile:306: common/arm/dct-a-8.o] Broken pipe
make: *** [Makefile:306: common/arm/deblock-a-8.o] Broken pipe
clang-10: error: unsupported argument '-mimplicit-it=always' to option 'Wa,'
make: *** [Makefile:306: common/arm/predict-a-8.o] Broken pipe
make: *** [Makefile:306: common/arm/pixel-a-8.o] Broken pipe
make: *** [Makefile:306: common/arm/quant-a-8.o] Broken pipe
Cleaning up file based variables
ERROR: Job failed: exit code 1