rustc: Allow using `clang` for wasm32 targets

rustc: Allow using `clang` for wasm32 targets

This commit adds support code for using `clang` directly to link the
wasm32-unknown-unknown target. Currently the target is only really
configured to link with LLD directly, but this ensures that `clang` can
be configured as well.

While not immediately useful in the near term it's likely that more
wasm32 targets will pop up over time with Clang's new native support for
WebAssembly in the 8.0.0 release. Getting support into rustc early
should make it easier to experiment with these targets and try out
various changes here and there.