cmd/compile: change sinit.go functions into methods

This will make it easier for subsequent CLs to track additional state
during package initialization scheduling.

Passes toolstash-check.

Updates #22326.

Change-Id: I528792ad34f41a4be52951531eb7525a94c9f350
Reviewed-on: https://go-review.googlesource.com/c/go/+/169898


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>