cmd/compile: rename init function from init.ializers back to init

The name change init -> init.ializers was initially required for
initialization code.

With CL 161337 there's no wrapper code any more, there's a data
structure instead (named .inittask). So we can go back to just
plain init appearing in tracebacks.

RELNOTE=yes

Update #29919. Followon to CL 161337.

Change-Id: I5a4a49d286df24b53b2baa193dfda482f3ea82a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/167780


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>