cmd/internal/gc: destutter array initializers

Aside from removing the superfluous comment near syms, this diff is
entirely mechanically generated via Emacs's query-replace-regexp to
replace "^\tstruct {\n[^}]*}" with "\t".

Change-Id: Ide7e4b5995f6a121b3f57415e033933ac5c7431a
Reviewed-on: https://go-review.googlesource.com/8427Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>