runtime: track time spent in mutator assists

This time is tracked per P and periodically flushed to the global
controller state. This will be used to compute mutator assist
utilization in order to schedule background GC work.

Change-Id: Ib94f90903d426a02cf488bf0e2ef67a068eb3eec
Reviewed-on: https://go-review.googlesource.com/8837
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/runtime2.go

@@ -366,6 +366,9 @@ type p struct {
 
 	palloc persistentAlloc // per-P to avoid mutex
 
+	// Per-P GC state
+	gcAssistTime int64 // Nanoseconds in assistAlloc
+
 	pad [64]byte
 }