runtime: fix sanity check in stackBarrier

stackBarrier on amd64 sanity checks that it's unwinding the correct
entry in the stack barrier array. However, this check is wrong in two
ways that make it unlikely to catch anything, right or wrong:

1) It checks that savedLRPtr == SP, but, in fact, it should be that
   savedLRPtr+8 == SP because the RET that returned to stackBarrier
   popped the saved LR. However, we didn't notice this check was wrong
   because,

2) the sense of the conditional branch is also wrong.

Fix both of these.

Change-Id: I38ba1f652b0168b5b2c11b81637656241262af7c
Reviewed-on: https://go-review.googlesource.com/17039Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/asm_amd64.s

@@ -378,8 +378,9 @@ TEXT runtime·stackBarrier(SB),NOSPLIT,$0
 	MOVQ	stkbar_savedLRPtr(DX)(BX*1), R8
 	MOVQ	stkbar_savedLRVal(DX)(BX*1), BX
 	// Assert that we're popping the right saved LR.
+	ADDQ	$8, R8
 	CMPQ	R8, SP
-	JNE	2(PC)
+	JEQ	2(PC)
 	MOVL	$0, 0
 	// Record that this stack barrier was hit.
 	ADDQ	$1, g_stkbarPos(CX)