runtime: insert itabs into hash table during init

runtime: insert itabs into hash table during init

See #14874

This change makes the runtime register all compiler generated itabs
(as obtained from the moduledata) during init.

Change-Id: I9969a0985b99b8bda820a631f7fe4c78f1174cdf
Reviewed-on: https://go-review.googlesource.com/20900
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Michel Lespinasse <walken@google.com>