[llvm] Prepend all library intrinsics with `#` when building for Arm64EC (PR #87542)

[Eli Friedman via llvm-commits]

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

LGTM, but we should probably look at adding more test coverage for this as a followup.

https://github.com/llvm/llvm-project/pull/87542