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

[Billy Laws via llvm-commits]

bylaws wrote:

@efriedma-quic  I should note there are several other fixes necessary for variadic calls in entry thunks to work properly, I pushed them a while back to https://github.com/bylaws/llvm-project/commit/4ff9104739ae1f8c9e411cd5eb32ae6562245382 but now entry thunks are merged I'll clean them up and PR.

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