[llvm-objdump] Exclude __mh_*_header symbols during MachO disassembly

`__mh_(execute|dylib|dylinker|bundle|preload|object)_header` are special symbols whose values hold the VMA of the Mach header to support introspection. They are attached to the first section in `__TEXT`, even though their addresses are outside `__TEXT`, and they do not refer to code.

It is normally harmless, but when the first section of `__TEXT` has no other symbols, `__mh_*_header` is considered by the disassembler when determing function boundaries. Since `__mh_*_header` refers to an address outside `__TEXT`, the boundary determination fails and disassembly quits.

Since `__TEXT,__text` normally has symbols, this bug is obscured. Experiments placing `__stubs` and `__stub_helper` first exposed the bug, since neither has symbols.

Differential Revision: https://reviews.llvm.org/D101786