[llvm] [AArch64] Fix variadic tail-calls on ARM64EC (PR #79774)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

The "1-byte" fixed object is maybe a little weird, but shouldn't cause any practical issue; really, it's a pointer to the end of the shadow, but that would just be more confusing.

I'd like to see a regression test that includes an opportunistic sibcall, in addition to the musttail call test.  (Maybe makes sense to test in a separate file, to make it easier to find stuff related to EC calling convention.)

https://github.com/llvm/llvm-project/pull/79774