runtime: fix stack corruption in race mode

MOVQ RARG0, 0(SP) smashes exactly what was saved by PUSHQ R15.
This code managed to work somehow with the current race runtime,
but corrupts caller arguments with new race runtime that I am testing.

Change-Id: I9ffe8b5eee86451db36e99dbf4d11f320192e576
Reviewed-on: https://go-review.googlesource.com/4810Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/race_amd64.s

@@ -383,8 +383,9 @@ TEXT	runtime·racesymbolizethunk(SB), NOSPLIT, $56-8
 	MOVQ	g_m(R13), R13
 	MOVQ	m_g0(R13), R14
 	MOVQ	R14, g(R12)	// g = m->g0
-	MOVQ	RARG0, 0(SP)	// func arg
+	PUSHQ	RARG0	// func arg
 	CALL	runtime·racesymbolize(SB)
+	POPQ	R12
 	// All registers are smashed after Go code, reload.
 	get_tls(R12)
 	MOVQ	g(R12), R13