[musttail] Don't forward AL on Win64
AL is only used for varargs on SysV platforms. Don't forward it on
Windows. This allows control flow guard to set up an extra hidden
parameter in RAX, as described in PR44049.
This also has the effect of freeing up RAX for use in virtual member
pointer thunks, which may also be a nice little code size improvement on
Reviewers: ajpaverd, efriedma, hans
Differential Revision: https://reviews.llvm.org/D70413