Add support for *-pc-windows-gnullvm targets

[AlexTMjugador]

These are tier 2 LLVM-based toolchains, so the LLVM prefix for windres should be used. I've tested these changes by compiling a "hello world" executable for the target in a Docker container and everything worked well, as far as I could see.

[AlexTMjugador]

Fixes #17. Because the *-pc-windows-gnullvm targets lack host tools, it can be assumed that cross-compilation (e.g., the host triple not being equal to the target triple) always happens.

[BenjaminRi]

Looks good to me. Thank you.

[ognevny]

it can be assumed that cross-compilation (e.g., the host triple not being equal to the target triple) always happens.

fyi, in MSYS2 all CLANG* environments use *-gnullvm host and target

[BenjaminRi]

The submitted change is still guarded by a check that we are cross-compiling. While my tests didn't reveal any breakage, please report related issues here if there are any.