Support a separate assembler tool, different from the normal C compiler
https://github.com/mesonbuild/meson/issues/4366
Also, in addition to this, meson strictly refuses to even try to compile .S
files with a Visual Studio-like compiler, which means that one can't easily use a wrapper that can call either cl.exe or armasm64.exe depending on file type, since meson knows that MSVC can't do that.