internal/trace: don't assume GC will start and end on same P

internal/trace: don't assume GC will start and end on same P

Currently, GC disables preemption between the traceGCStart and
traceGCDone, so it never moves Ps. Consequently, the trace verifier
attaches information about GC to its per-P state and will fail if GC
starts on one P and ends on another.

GC will soon be preemptible and may end on a different P than it
began. Hence, this change lifts this per-P verifier state to global
state.

Change-Id: I82256e2baab1ff3c4453fec312079018423b4b51
Reviewed-on: https://go-review.googlesource.com/8714
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>