Trampolines to off-heap targets may get inlined. In this case, it is hard
to tell where the trampoline is going to; this CL adds code comments which
identify the target builtin by name.

Bug: v8:6666
Change-Id: Ib085ad118ad64551af2522f9187b4faaad6ce315
Reviewed-on: https://chromium-review.googlesource.com/1160536
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54874}