-rpath argument passed even when disabled in target

[tlively]

This occurs specifically when building libcore, libstd, liballoc, etc. for Emscripten, as observed in #63649. A workaround was added upstream in emscripten-core/emscripten#9475, but we would be able to remove that workaround if this issue were fixed in the rust build.

[tlively]

I found the underlying issue, which was that src/bootstrap/builder.rs was explicitly adding the rpath argument. I just had to add emscripten to the list of targets for which it should not do that. This has been done as part of #63649.