runtime: add new modtimer function

This adds a new field to P, adjustTimers, that tells the P that one of
its existing timers was modified to be earlier, and that it therefore
needs to resort them.

Updates #27707

Change-Id: I4c5f5b51ed116f1d898d3f87cdddfa1b552337f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/171832
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/runtime2.go

@@ -607,6 +607,12 @@ type p struct {
 	// Must hold timersLock to access.
 	timers []*timer
 
+	// Number of timerModifiedEarlier timers on P's heap.
+	// This should only be modified while holding timersLock,
+	// or while the timer status is in a transient state
+	// such as timerModifying.
+	adjustTimers uint32
+
 	pad cpu.CacheLinePad
 }