runtime: fix sigprof frame counting

If sigtramp and sigtrampgo are both on stack, n -= framesToDiscard
    is executed twice, which should actually run only once.
    
    Reviewed-on: https://go-review.googlesource.com/c/159238

From-SVN: r268366

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-c2cac0ba0a92e74d5675c3c9f4e53d2567dbc903
+5af8ee0693944c280b1f529450dbfd4ec1ee451d
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/proc.go

@@ -3542,14 +3542,13 @@ func sigprof(pc uintptr, gp *g, mp *m) {
 		for i := 0; i < n; i++ {
 			if stklocs[i].function == "runtime.sigtrampgo" && i+2 < n {
 				framesToDiscard = i + 2
-				n -= framesToDiscard
 			}
 			if stklocs[i].function == "runtime.sigtramp" && i+2 < n {
 				framesToDiscard = i + 2
-				n -= framesToDiscard
 				break
 			}
 		}
+		n -= framesToDiscard
 		for i := 0; i < n; i++ {
 			stk[i] = stklocs[i+framesToDiscard].pc
 		}