llvm-objdump should ignore Mach-O stab symbols for disassembly.

Summary:
llvm-objdump will commonly error out when disassembling a Mach-O binary with
stab symbols, or when printing a Mach-O symbol table that includesstab symbols.
That is because the Mach-O N_OSO symbol has been modified to include the
bottom 8-bit value of the Mach-O's cpusubtype value in the section field. In
general, one cannot blindly assume a stab symbol's section field is valid
unless one has actually consulted the specification for the specific stab.

Since objdump mostly just walks the symbol table to get mnemonics for code
disassembly it's best for objdump to just ignore stab symbols. llvm-nm will
do a more complete and correct job of displaying Mach-O symbol table contents.

Reviewers: pete, lhames, ab, thegameg, jhenderson, MaskRay

Reviewed By: thegameg, MaskRay

Subscribers: MaskRay, rupprecht, seiya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71394