runtime: fix arm build (B not JMP)

TBR=dvyukov
CC=golang-codereviews
https://golang.org/cl/71060046
diff --git a/src/pkg/runtime/asm_arm.s b/src/pkg/runtime/asm_arm.s
index aa171d7..3aed51f 100644
--- a/src/pkg/runtime/asm_arm.s
+++ b/src/pkg/runtime/asm_arm.s
@@ -215,7 +215,7 @@
 
 TEXT runtime·morestack_noctxt(SB),NOSPLIT,$-4-0
 	MOVW	$0, R7
-	JMP runtime·morestack(SB)
+	B runtime·morestack(SB)
 
 // Called from panic.  Mimics morestack,
 // reuses stack growth code to create a frame