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
runtime: remove singleproc var
It was needed for the old scheduler,
because there temporary could be more threads than gomaxprocs.
In the new scheduler gomaxprocs is always respected.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/12438043