runtime: prevent race between setNextBarrierPC and sigprof

Currently, setNextBarrierPC manipulates the stack barriers without
acquiring the stack barrier lock. This is mostly okay because
setNextBarrierPC also runs synchronously on the G and prevents safe
points, but this doesn't prevent a sigprof from occurring during a
setNextBarrierPC and performing a traceback.

Given that setNextBarrierPC simply sets one entry in the stack barrier
array, this is almost certainly safe in reality. However, given that
this depends on a subtle argument, which may not hold in the future,
and that setNextBarrierPC almost never happens, making it nowhere near
performance-critical, we can simply acquire the stack barrier lock and
be sure that the synchronization will work.

Updates #12528. For 1.5.3.

Change-Id: Ife696e10d969f190157eb1cbe762a2de2ebce079
Reviewed-on: https://go-review.googlesource.com/18022
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mstkbar.go

@@ -305,7 +305,9 @@ func nextBarrierPC() uintptr {
 //go:nosplit
 func setNextBarrierPC(pc uintptr) {
 	gp := getg()
+	gcLockStackBarriers(gp)
 	gp.stkbar[gp.stkbarPos].savedLRVal = pc
+	gcUnlockStackBarriers(gp)
 }
 
 // gcLockStackBarriers synchronizes with tracebacks of gp's stack