[llvm-objcopy][MachO] Handle relocation entries where r_extern is zero

Fix handling of relocations with r_extern == 0.
If r_extern == 0 then r_symbolnum is an index of a section rather than a symbol index.

Patch by Seiya Nuta and Alexander Shaposhnikov.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D78946