[llvm] Reland [AArch64] Copy x4/x5 vararg payload into the x64 stack in Arm64EC exit thunks (PR #208453)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic approved this pull request.

Maybe add -verify-machineinstrs to the RUN lines of the tests so we catch issues earlier in the future.  Otherwise LGTM

https://github.com/llvm/llvm-project/pull/208453