[llvm] [AArch64] Disable variadic tail-calls for ARM64EC (PR #78914)

[Billy Laws via llvm-commits]

bylaws wrote:


> "Current SP" is meaningful for a tail-call; it's just the SP on entry. I guess the current code doesn't compute that correctly, but it's very easy to fix; we do a similar computation elsewhere (see https://github.com/llvm/llvm-project/blob/6c98c5bd99b4c71a7895337cd4e437e023c2ec7a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp#L7951).
Oh thanks for the pointer, will writeup a fix with that instead. 


https://github.com/llvm/llvm-project/pull/78914