cmd/gc: fix warning on Plan 9

warning: src/cmd/gc/bits.c:101 non-interruptable temporary

Change-Id: I74661fefab50455b912b8085d913fc45ba13c5c8
Reviewed-on: https://go-review.googlesource.com/1780
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>