[mach-o] Fix initial live atoms with -dead_strip

When -dead_strip is used with -exported_symbols_list the initial set of
live atoms are those in the export list.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@216213 91177308-0d34-0410-b5e6-96231b3b80d8