[AArch64] Preserve X8 for thunks ending in variadic musttail calls
On Windows, X8 may be used to pass in the address of an aggregate that
is returned indirectly. Therefore, it should be forwarded to variadic
musttail calls and preserved in thunks.
Reviewers: mgrang, efriedma
Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D62344