cmd/{5,6,8,9}g, cmd/internal/gc: use bools for is* and okfor*

cmd/{5,6,8,9}g, cmd/internal/gc: use bools for is* and okfor*

No functional changes.

This diff was generated as follows:

* Manually edit cmd/internal/gc/go.go to update types and group variables.
* Manually edit initialization in cmd/internal/gc/align.go--localized s/1/true.
* Manually fix the handling of sign in cmd/internal/gc/walk.go in func bounded (near line 4000).
* Manually update go.y and regenerate y.go.
* Run gofmt -r many times to do the rest, using https://gist.github.com/josharian/0f61dbb2dff81f938e70.

toolstash -cmp on the stdlib comes back green.

Change-Id: I19766ed551714e51b325133e7138818d117b3a9a
Reviewed-on: https://go-review.googlesource.com/6530
Reviewed-by: Russ Cox <rsc@golang.org>