[mach-o] refactor mach-o output section selection to be table driven.

In -r mode the sections use the table used to parse .o files.  Otherwise
use final exectuable table.  No functionality change.



git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@210506 91177308-0d34-0410-b5e6-96231b3b80d8