cmd/compile/internal/gc: clean up sinit.go

cmd/compile/internal/gc: clean up sinit.go

The ctxt parameter is always set to 0 on entry into anylit so make this
parameter a literal constant, and where possibly remove ctxt as a parameter
where it is known to be a constant zero.

Change-Id: I3e76e06456d7b1a1ea875ffeb2efefa4a1ff5a7e
Reviewed-on: https://go-review.googlesource.com/28221
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>