[MachO] Port call graph profile section and directive

This ports the `.cg_profile` assembly directive and call graph profile section
generation to MachO from COFF/ELF. Due to MachO section naming rules, the
section is called `__LLVM,__cg_profile` rather than `.llvm.call-graph-profile`
as in COFF/ELF. Support for llvm-readobj is included to facilitate testing.

Corresponding LLD change is D112164

Differential Revision: https://reviews.llvm.org/D112160