WIP: stdcall name mangling

ref #7375
@Keno I'm using llvm::Mangler to compute the mangled name for this function signature. However, do you have any idea how the mangled name (for asm) would map back to the name expected by `dlsym`, in general? For example, on OSX, `dlsym` unconditionally adds an `_` (https://www.opensource.apple.com/source/dyld/dyld-239.4/src/dyldAPIs.cpp). But then there's also code in Mangler to handle utf8 characters and such and mangle them appropriately for the .s file.