Linker plugin LTO

Rust currently supports [linker plugin LTO](https://doc.rust-lang.org/rustc/linker-plugin-lto.html) (aka cross language optimization, aka link time optimization).

It would be interesting to see impact of this in the context of OpenSSL. Basically compiling OpenSSL with clang `-flto`, so it will allow cross inlining and dead code elimination, etc.. I'm hoping that this will greatly reduce size of the resulting executable.