Rust version of `__attribute(used)__` (GCC) or `llvm.used` (LLVM).

There come times when rust symbols are referenced only by external assembler. It seems rustc is eager to remove them (especially with `-C lto`, even making them public does not help).

In clang and gcc there are ways to prevent it, correspondingly: `__attribute__((used))` and ` `llvm.used`

If there's a way to get the same behavior in Rust? If not, can be added?
