Rollup merge of #44542 - alexcrichton:fix-windows-emscripten, r=nikomatsakis

rustc: Spawn `cmd /c emcc.bat` explicitly

In #42436 the behavior for spawning processes on Windows was tweaked slightly to
fix various bugs, but this caused #42791 as a regression, namely that to spawn
batch scripts they need to be manually spawned with `cmd /c` instead now. This
updates the compiler to handle this case explicitly for Emscripten.

Closes #42791