[lld-macho] Add -pagezero_size

Adds `-pagezero_size`. `-pagezero_size` commonly used for kernel development.
`-pagezero_size` changes the `__PAGEZERO` size, removing that segment if it is set to zero.

One of the four flags from {D118570}
Now with error messages and tests.

Differential Revision: https://reviews.llvm.org/D118724