Add generation of LC_VERSION_MIN load commands.

If the command line contains something like -macosx_version_min and we
don't explicitly disable generation with -no_version_load_command then
we generate the LC_VERSION_MIN command in the output file.

There's a couple of FIXME's in here.  These will be handled soon with
more tests but I didn't want to grow this patch any more than it already was.

rdar://problem/24472630

llvm-svn: 259718