[mach-o] update __eh_frame handling for Nick's suggestions

First, add a comment to support more variation in FDE formats. Second, refactor
fde -> function handling into a separate function living in the ArchHandler.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@219959 91177308-0d34-0410-b5e6-96231b3b80d8