runtime: fix stack switch check in walltime/nanotime on linux/arm

CL 98095 got the check wrong. We should be testing
'getg() == getg().m.curg', not 'getg().m == getg().m.curg'.

Change-Id: I32f6238b00409b67afa8efe732513d542aec5bc7
Reviewed-on: https://go-review.googlesource.com/98855
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/sys_linux_arm.s

@@ -221,7 +221,7 @@ TEXT runtime·walltime(SB),NOSPLIT,$0-12
 	MOVW	g_m(g), R1
 	MOVW	m_curg(R1), R0
 
-	CMP	R1, R0		// Only switch if on curg.
+	CMP	g, R0		// Only switch if on curg.
 	B.NE	noswitch
 
 	MOVW	m_g0(R1), R0
@@ -266,7 +266,7 @@ TEXT runtime·nanotime(SB),NOSPLIT,$0-8
 	MOVW	g_m(g), R1
 	MOVW	m_curg(R1), R0
 
-	CMP	R1, R0		// Only switch if on curg.
+	CMP	g, R0		// Only switch if on curg.
 	B.NE	noswitch
 
 	MOVW	m_g0(R1), R0