[llvm-objcopy][MachO] Skip sections with zero offset

[llvm-objcopy][MachO] Skip sections with zero offset

Some binaries can contain regular sections with zero offset and zero size.
This diff makes llvm-objcopy's handling of such sections consistent with
cctools's strip (which doesn't modify them),
previously the tool would allocate file space for them.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D90796