[llvm-objcopy][MachO] Fix symbol order in the symbol table

Only consider isUndefinedSymbol() when the symbol is not local. This
fixes an assert failure when copying the symbol table, if a n_type=0x20
symbol is followed by a n_type=0x64 symbol.

Reviewed By: alexshap, seiya

Differential Revision: https://reviews.llvm.org/D70475