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
Alexander Shaposhnikov