[PATCH] mscoree: Implement VTable fixup for x86_64 architecture.

[Vincent Povirk]

I actually think it's very important to get theading right, even if
it's a "theoretical issue" that "probably won't happen".

A managed C++ project probably will not do its runtime initialization
from multiple threads, but it could have multiple threads accessing
another vtable later. Vtable fixups could also be used to export
functions to a mostly unmanaged program that expects them to be
thread-safe.

I still believe this function is safe, but if it's not then I want to
address it.

Mono has a lock around the JIT compiler, and it caches the functions
it generates. So we can have multiple threads writing the same
function pointers into the vtable (which should be fine as long as
every other thread that reads it will see either the old value or the
new one). Mono doesn't reference-count JITed functions, and I'm not
sure if cleaning them up is even possible.

Non-locked write to fixup->done is also OK as long as no thread can
incorrectly read the value as TRUE and return before the vtable is
ready. If a thread incorrectly reads it as FALSE, it'll just do a bit
of unnecessary work. Actually, I'm not sure that field is needed at
all, it seems like overwriting the values in the vtable should be
enough.