runtime: don't use R11 in nanotime1/walltime1 on ARM

R11 a.k.a. REGTMP is the temp register used by the assembler. It
may be clobbered if the assembler needs to synthesize
instructions. In particular, in nanotime1/walltime1, the load of
global variable runtime.iscgo clobbers it. So, avoid using R11
to hold a long-lived value.

Fixes #36309.

Change-Id: Iec2ab9d664532cad8fbf58da17f580e64a744f62
Reviewed-on: https://go-review.googlesource.com/c/go/+/212641Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Andrew G. Morgan <agm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/sys_linux_arm.s

@@ -269,8 +269,8 @@ noswitch:
 
 	MOVW	$CLOCK_REALTIME, R0
 	MOVW	$8(R13), R1	// timespec
-	MOVW	runtime·vdsoClockgettimeSym(SB), R11
-	CMP	$0, R11
+	MOVW	runtime·vdsoClockgettimeSym(SB), R2
+	CMP	$0, R2
 	B.EQ	fallback
 
 	// Store g on gsignal's stack, so if we receive a signal
@@ -292,7 +292,7 @@ noswitch:
 	MOVW	(g_stack+stack_lo)(R6), R6 // g.m.gsignal.stack.lo
 	MOVW	g, (R6)
 
-	BL	(R11)
+	BL	(R2)
 
 	MOVW	$0, R1
 	MOVW	R1, (R6) // clear g slot, R6 is unchanged by C code
@@ -300,7 +300,7 @@ noswitch:
 	JMP	finish
 
 nosaveg:
-	BL	(R11)
+	BL	(R2)
 	JMP	finish
 
 fallback:
@@ -347,8 +347,8 @@ noswitch:
 
 	MOVW	$CLOCK_MONOTONIC, R0
 	MOVW	$8(R13), R1	// timespec
-	MOVW	runtime·vdsoClockgettimeSym(SB), R11
-	CMP	$0, R11
+	MOVW	runtime·vdsoClockgettimeSym(SB), R2
+	CMP	$0, R2
 	B.EQ	fallback
 
 	// Store g on gsignal's stack, so if we receive a signal
@@ -370,7 +370,7 @@ noswitch:
 	MOVW	(g_stack+stack_lo)(R6), R6 // g.m.gsignal.stack.lo
 	MOVW	g, (R6)
 
-	BL	(R11)
+	BL	(R2)
 
 	MOVW	$0, R1
 	MOVW	R1, (R6) // clear g slot, R6 is unchanged by C code
@@ -378,7 +378,7 @@ noswitch:
 	JMP	finish
 
 nosaveg:
-	BL	(R11)
+	BL	(R2)
 	JMP	finish
 
 fallback: