Consider use of @platforms//cpu:wasm32 instead of just "wasm" for Bazel toolchain

In https://github.com/emscripten-core/emsdk/blob/85d27a4a2a60d591613a305b14ae438c2bb3ce11/bazel/emscripten_toolchain/BUILD.bazel#L73-L74
the supported cpu architecture is labeled "wasm". In [bazelbuild/platforms/cpu](https://github.com/bazelbuild/platforms/blob/e658a6af526089406d0057160542597501ba65d7/cpu/BUILD#L109), they define the cpu constant as "wasm32".

Should we modify the toolchain parameters to 1) accept that value and 2) change the uses of "wasm" in those Bazel rules to "wasm32"? I'm still a little new to the Bazel ecosystem, but my understanding is that would let us be more aligned with the broader ecosystem.

I'm also not clear if that's a simple fix or if there are some other places that would need updating, otherwise I would have sent a PR instead of an issue.