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

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang