Applies to API implementation of O.p.toString

- Removes special handling of non-string @@toStringTag values (use builtinTag)
- Removes special handling of @@toStringTags which match [[Class]] names (remove ~ prefix)
- Add tests for non-string @@toStringTag values in test-api.cc

BUG=v8:3502
R=arv@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/892393003

Cr-Commit-Position: refs/heads/master@{#26415}