runtime: revert UseSpanType back to 1
runtime: revert UseSpanType back to 1

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/7812043

gnanderson/go,gnanderson/go,tv42/old-go,apparentlymart/golang,dsriram/golang,gnanderson/go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,dsriram/golang,dsriram/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,dsriram/golang