Fix llvm-objdump when disassembling a stripped Mach-O binary with the -macho option.

It was printing out nothing in this case.

llvm-objdump tries to disassemble sections a symbol at a time.  In the case of a
fully stripped Mach-O executable the only symbol remaining in the (__TEXT,__text)
section is the special linker defined symbol __mh_execute_header . This
symbol is special in that while it is N_SECT symbol in the (__TEXT,__text)
its address is before the start of the (__TEXT,__text).  It’s address is the
start of the __TEXT segment which is where the mach header is statically
linked. So the code in DisassembleMachO() needs to deal with this case specially.

rdar://26778273

llvm-svn: 272837