Use Clang to compile NSIS

We would like to drop GCC on Windows (#29318 (closed)), however we still have two projects that use it: Rust (#29320 (closed)) and NSIS.

NSIS can be mostly compiled with Clang (even though we need to manually link $arch-w64-mingw-gcc and $arch-w64-mingw-g++ to $arch-w64-mingw-clang and $arch-w64-mingw-clang++, respectively).

The stopper is that NSIS contains 2 or 3 assembly files with some awkward/proprietary syntax, which are recognized neither by clang, nor by GNU as.

Mozilla uses the trick to ask Clang to use as (with -fno-integrated-as), but that doesn't work for us. I haven't understood why, does Firefox contain another assembler? NASM does not count, because it uses Intel syntax, our files are in AT&T syntax. In any case, see taskcluster/scripts/misc/build-mingw32-nsis.sh for more information.

Anyway, I think we'll have to keep GCC for Rust at least until we switch to LLVM 15, so we may just keep GCC also for NSIS, and deal with it later.