Unify mingw-w64-clang 32+64 bits

Clang has the advantage over GCC that you can use -taget at runtime.

Indeed, we don't recompile the whole Clang, but only a few binaries, like mingw and the runtimes (compiler-rt, libcxx and libcxxabi).

So, we could merge the two archives, and replace them with a single one, with a slightly higher size (the merged directory is 2.8GB uncompressed, vs. 2.7GB of each the single-arch mingw-w64-clang directories uncompressed, and the merged archive is 825MB vs two archives of 812MB).

The advantage is that we'd have both the 32-bit and 64-bit always available, if needed for some projects (I don't think any need, at the moment).

The build script itself is already quite parametric, we could replace it with a build() bash function that takes the architecture as an input parameter.

Diff of the directories

The screenshot shows that the two toolchain are completely overlappable.