[llvm] [llvm-size] Add -z option for Mach-O to exclude __PAGEZERO size. (PR #159574)

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

Please make sure to update the llvm-size command guide (llvm/docs/CommandGuide/llvm-size.rst) with the new option.

This will need a Mach-O developer to review, since I don't know the intricacies of the segment layout for these files. @drodriguez / @danzimm / @alexander-shaposhnikov, can any of you help? 

https://github.com/llvm/llvm-project/pull/159574