Band-aid fix for infinite recursion in RegExp TFJ builtins.

TFJ builtins don't contain stack checks in general, so any deep
recursion involving only TFJ builtins can end up overflowing the stack
and segfaulting on the red area.

RegExp builtins in particular can only build such recursions using
RegExp.p.exec, and (as far as I can tell) only by modifying the instance
or prototype, thus hitting the slow path in all builtins.

This CL adds a stack check to RegExpExec, which is the choke point for
calling exec on slow-mode RegExps.

Bug: v8:7239, chromium:797481

Regression test

Change-Id: I78dbb5f868a775d9697606d513623f912639d7db
Reviewed-on: https://chromium-review.googlesource.com/856777Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50511}