cmd/compile: create "init" symbol earlier

We create the "init" symbol and mark it as a function before compiling
to SSA because SSA can initialize this symbol, but it turns out we do
it slightly too late. peekitabs, at least, can also create the "init"
LSym. Move this initialization to just after type-checking.

Fixes the linux-amd64-ssacheck and the android-arm64-wiko-fever
builders.

Updates #27539.

Change-Id: If145952c79d39f75c93b24e35e67fe026dd08329
Reviewed-on: https://go-review.googlesource.com/c/149137


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>