Add support for leaf function frame pointer elimination
This PR adds ability for the target specifications to specify frame
pointer emission type that's not just “always” or “whatever cg decides”.
In particular there's a new mode that allows omission of the frame
pointer for leaf functions (those that don't call any other functions).
We then set this new mode for Aarch64-based Apple targets.
Fixes#86196
dfd30d7b70