This ports the platform-specific SubStringStub to TurboFan.

It also contains a minor bug-fix for the case when the requested substring
length equals the subject string length, but the start index is not equal to 0.
The old stub implementation returned the subject string, while the new
implementation calls into runtime, which finally results in a thrown exception.

BUG=v8:5415

Review-Url: https://codereview.chromium.org/2355793003
Cr-Commit-Position: refs/heads/master@{#39653}