runtime: remove memset/memclr shim

This CL resolves https://golang.org/cl/6300043/#msg3
by renaming memset_arm.s to memclr_arm.s and merging the function
of the same name from asm_arm.s.

R=minux.ma, rsc
CC=golang-dev
https://golang.org/cl/6336054

src/pkg/runtime/asm_arm.s

@@ -345,13 +345,6 @@ TEXT	runtime·cgocallback(SB),7,$16
 	// Done!
 	RET
 
-TEXT runtime·memclr(SB),7,$0
-	MOVW	0(FP), R0
-	MOVW	$0, R1
-	MOVW	R1, 0(FP)
-	BL	runtime·memset(SB)
-	RET
-
 TEXT runtime·getcallerpc(SB),7,$-4
 	MOVW	0(SP), R0
 	RET

src/pkg/runtime/memset_arm.s → src/pkg/runtime/memclr_arm.s

@@ -28,10 +28,10 @@ TOE = 11
 N = 12
 TMP = 12				/* N and TMP don't overlap */
 
-TEXT runtime·memset(SB), $0
-	MOVW	R0, R(TO)
-	MOVW	data+4(FP), R(0)
-	MOVW	n+8(FP), R(N)
+TEXT runtime·memclr(SB),7,$0
+	MOVW	ptr+0(FP), R(TO)
+	MOVW	n+4(FP), R(N)
+	MOVW	$0, R(0)
 
 	ADD	R(N), R(TO), R(TOE)	/* to end pointer */