devel/llvm-{cheri*,morello}: new snapshots

Update llvm-cheri to a new LLVM 17 based snapshot, llvm-cheriot to an
LLVM 18 based snapshot, and llvm-morello to a snapshot with new function
pointer semantics to better support compartmentalization.

Update the wrapper script to enable the new function pointer semantics
on sufficently new CheriBSD system.

Sponsored by: DARPA, AFRL