[mach-o] Improve x86_64 reference kind switch statements.

The use of default: was disabling the warning about unused enumerators. Fix
that, then fix the one enumerator that was not handled. Add coverage for
it in test suite.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217078 91177308-0d34-0410-b5e6-96231b3b80d8