[COFF, ARM64] Make sure to forward arguments from vararg to musttail vararg

Summary:
    Thunk functions in Windows are varag functions that call a musttail function
    to pass the arguments after the fixup is done.  We need to make sure that we
    forward the arguments from the caller vararg to the callee vararg function.
    This is the same mechanism that is used for Windows on X86.

Reviewers: ssijaric, eli.friedman, TomTan, mgrang, mstorsjo, rnk, compnerd, efriedma

Reviewed By: efriedma

Subscribers: efriedma, kristof.beyls, chrib, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D53843

llvm-svn: 345641