command line passthrough is flawed

[nagisa]

$ rustc test.rs --emit=llvm-ir,asm --crate-type=lib -O -o test
# works
$ rustup run beta rustc test.rs --emit=llvm-ir,asm --crate-type=lib -O -o test
error: multiple input filenames provided
$ rustup run beta rustc -- test.rs --emit=llvm-ir,asm --crate-type=lib -O -o test
error: multiple input filenames provided
$ rustup run beta -- rustc test.rs --emit=llvm-ir,asm --crate-type=lib -O -o test
error: multiple input filenames provided

rustup assumes --emit=llvm-ir,asm is not a single argument for some reason and splits --emit=llvm-ir,asm on the comma, passing rustc two distinct arguments.

[Diggsey]

Dup of #665