[mach-o]: atomize zero-terminated literals correctly.

When looking through sections with zero-terminated string-literals (__cstring
or __ustring) we were constantly rechecking the first few bytes of the string
for '\0' rather than advancing along. This obviously failed unless all strings
within the section had the same length as that first one.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@211682 91177308-0d34-0410-b5e6-96231b3b80d8