remove LSDA pointer from EhFrameHeader

[WalterBright]

This starts the transition to Apple's left turn on the Mach object file format:

1. removed LSDA reference from EhFrameHeader
2. adds comments about compact_unwind_entry format
3. makes comdat functions now with N_WEAK_DEF instead of __textcoal_nt section

Sorry this won't be easy to review if not familiar with the undocumented Apple Arm64 object file format.

[Herringway]

Maybe they SHOULD document it, but it doesn't surprise me that they don't document a haphazardly fast-moving target. I'm sure whatever DMD is implementing today will be wrong by the next release.

[WalterBright]

The object file format cannot be a haphazard fast moving target, as too many projects depend on it working as expected.

[Herringway]

The object file format cannot be a haphazard fast moving target, as too many projects depend on it working as expected.

The projects can adapt. It's not Apple's problem.

[ibuclaw]

Textcoal change ought to have been separate PR, also closes this issue #18063