undo CL 12250043 / e911f94c4902

Break all 386 builders.

??? original CL description
runtime: use gcpc/gcsp during traceback of goroutines in syscalls
gcpc/gcsp are used by GC in similar situation.
gcpc/gcsp are also more stable than gp->sched,
because gp->sched is mutated by entersyscall/exitsyscall
in morestack and mcall. So it has higher chances of being inconsistent.
Also, rename gcpc/gcsp to syscallpc/syscallsp.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/12250043
???

R=rsc
CC=golang-dev
https://codereview.appspot.com/12424045