rustc: Add --target-cpu flag to select a more specific processor instead of the default, 'generic'.
auto merge of #8410 : luqmana/rust/mcpu, r=sanxiyn

Adds `--target-cpu` flag which lets you choose a more specific target cpu instead of just passing the default, `generic`. It's more or less akin to `-mcpu`/`-mtune` in clang/gcc.