[mach-o]: implement -image_base option on Darwin.

As suggested by Nick, this will make __unwind_info implementation more natural,
and it'd have to be done at some point anyway.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217486 91177308-0d34-0410-b5e6-96231b3b80d8