[release-branch.go1.8] runtime: avoid O(n) semaphore list walk in contention profiling

Contention profiling is off by default.
If you turn it on, it has the unfortunate effect of making
the wakeup on a contention mutex go from O(1) to O(n).
Change it back to O(1).

This is already fixed in essentially the same way on master;
master also contains some fixes for the non-profiling code
paths.

Possible for Go 1.8.1.

Change-Id: Iaa644c06e20ca28da4dfa348b7211eedb657e0ba
Reviewed-on: https://go-review.googlesource.com/37341
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/sema.go

@@ -171,6 +171,7 @@ func semrelease(addr *uint32) {
 			for x := root.head; x != nil; x = x.next {
 				if x.elem == unsafe.Pointer(addr) {
 					x.acquiretime = t0
+					break
 				}
 			}
 			mutexevent(t0-s.acquiretime, 3)