cmd/8g, cmd/dist, cmd/gc: fix warnings on Plan 9

cmd/8g/gsubr.c: unreachable code
cmd/8g/reg.c: overspecifed class
cmd/dist/plan9.c: unused parameter
cmd/gc/fmt.c: stkdelta is now a vlong
cmd/gc/racewalk.c: used but not set

R=golang-dev, seed, rsc
CC=golang-dev
https://codereview.appspot.com/7067052