[llvm-objcopy][MachO] Fix segment size alignment

This diff addresses the old TODO in MachOObjcopy.cpp and
correctly sets the page size used for alignment of segments.
In particular, now llvm-objcopy's output is consistent
with the input (the alignment of vmsize doesn't change).

Test plan:

1. make check-all
2. verify that a binary copied via llvm-objcopy now correctly works on iOS.

Differential revision: https://reviews.llvm.org/D82503