runtime: replace lock() with casp() in the GC
runtime: replace lock() with casp() in the GC

Note: BitTarget will be removed by a forthcoming changeset.

R=golang-dev, dvyukov
CC=golang-dev, rsc
https://codereview.appspot.com/7837044

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