[AArch64] Preserve X8 for thunks ending in variadic musttail calls

Summary:
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.

Fixes PR41997

Reviewers: mgrang, efriedma

Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62344

llvm-svn: 361585