[llvm-readobj] Add support for dumping MachO min version load command.

Example output:
File: <stdin>
Format: Mach-O arm
Arch: arm
AddressSize: 32bit
MinVersion {
  Cmd: LC_VERSION_MIN_IPHONEOS
  Size: 16
  Version: 99.8.7
  SDK: n/a
}

Differential Revision:	 http://reviews.llvm.org/D12373

llvm-svn: 246151