big: fix arm build - forgot how to spell JMP

Fixes #269.

TBR=kaib
https://golang.org/cl/156107

src/pkg/big/arith_arm.s

@@ -7,23 +7,23 @@
 
 // TODO(gri) Implement these routines.
 TEXT big·addVV_s(SB),7,$0
-	JMP big·addVV_g(SB)
+	B big·addVV_g(SB)
 
 TEXT big·subVV_s(SB),7,$0
-	JMP big·subVV_g(SB)
+	B big·subVV_g(SB)
 
 TEXT big·addVW_s(SB),7,$0
-	JMP big·addVW_g(SB)
+	B big·addVW_g(SB)
 
 TEXT big·subVW_s(SB),7,$0
-	JMP big·subVW_g(SB)
+	B big·subVW_g(SB)
 
 TEXT big·mulAddVWW_s(SB),7,$0
-	JMP big·mulAddVWW_g(SB)
+	B big·mulAddVWW_g(SB)
 
 TEXT big·addMulVVW_s(SB),7,$0
-	JMP big·addMulVVW_g(SB)
+	B big·addMulVVW_g(SB)
 
 TEXT big·divWVW_s(SB),7,$0
-	JMP big·divWVW_g(SB)
+	B big·divWVW_g(SB)