runtime: simplify walltime/nanotime on linux/{386,amd64}

Avoid an unnecessary MOVL/MOVQ.

Follow CL 97377

Change-Id: Ic43976d6b0cece3ed455496d18aedd67e0337d3f
Reviewed-on: https://go-review.googlesource.com/97358
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/sys_linux_386.s

@@ -203,9 +203,8 @@ TEXT runtime·walltime(SB), NOSPLIT, $0-12
 	get_tls(CX)
 	MOVL	g(CX), AX
 	MOVL	g_m(AX), CX
-	MOVL	m_curg(CX), DX
 
-	CMPL	AX, DX		// Only switch if on curg.
+	CMPL	AX, m_curg(CX)	// Only switch if on curg.
 	JNE	noswitch
 
 	MOVL	m_g0(CX), DX
@@ -260,9 +259,8 @@ TEXT runtime·nanotime(SB), NOSPLIT, $0-8
 	get_tls(CX)
 	MOVL	g(CX), AX
 	MOVL	g_m(AX), CX
-	MOVL	m_curg(CX), DX
 
-	CMPL	AX, DX		// Only switch if on curg.
+	CMPL	AX, m_curg(CX)	// Only switch if on curg.
 	JNE	noswitch
 
 	MOVL	m_g0(CX), DX

src/runtime/sys_linux_amd64.s

@@ -189,9 +189,8 @@ TEXT runtime·walltime(SB),NOSPLIT,$0-12
 	get_tls(CX)
 	MOVQ	g(CX), AX
 	MOVQ	g_m(AX), CX
-	MOVQ	m_curg(CX), DX
 
-	CMPQ	AX, DX		// Only switch if on curg.
+	CMPQ	AX, m_curg(CX)	// Only switch if on curg.
 	JNE	noswitch
 
 	MOVQ	m_g0(CX), DX
@@ -234,9 +233,8 @@ TEXT runtime·nanotime(SB),NOSPLIT,$0-8
 	get_tls(CX)
 	MOVQ	g(CX), AX
 	MOVQ	g_m(AX), CX
-	MOVQ	m_curg(CX), DX
 
-	CMPQ	AX, DX		// Only switch if on curg.
+	CMPQ	AX, m_curg(CX)	// Only switch if on curg.
 	JNE	noswitch
 
 	MOVQ	m_g0(CX), DX