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

GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust