[macho2yaml] Add support for dumping mach_headers

This patch adds the ability to dump mach headers. For my local clang binary the macho2yaml output is now:

--- !mach-o
FileHeader:
  cputype:         0x01000007
  cpusubtype:      0x80000003
  filetype:        0x00000002
  ncmds:           19
  flags:           0x00A18085
...

llvm-svn: 269304