[llvm-readobj] MachO: Dump segment command.

Example output:

File: <stdin>
Format: Mach-O arm
Arch: arm
AddressSize: 32bit
Segment {
  Cmd: LC_SEGMENT
  Name:
  Size: 260
  vmaddr: 0x0
  vmsize: 0x10
  fileoff: 408
  filesize: 408
  maxprot: rwx
  initprot: rwx
  nsects: 3
  flags: 0x0
}

Differential Revision:	 http://reviews.llvm.org/D12542

llvm-svn: 246665