`-Cstack-alignment`/`-Cpreferred-stack-boundary` support in `rustc`

i.e. the equivalent of `-mpreferred-stack-boundary` ([GCC](https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html#index-mpreferred-stack-boundary-1), Clang does not support it) / `-mstack-alignment` (GCC does not support it, [Clang](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mstack-alignment)).