in arm cancellation point asm, don't unnecessarily preserve link register

The only reason we needed to preserve the link register was because we
were using a branch-link instruction to branch to __cp_cancel.
Replacing this with a branch means we can avoid the save/restore as
the link register is no longer modified.

src/thread/arm/syscall_cp.s

@@ -11,19 +11,19 @@
 .type __syscall_cp_asm,%function
 __syscall_cp_asm:
 	mov ip,sp
-	stmfd sp!,{r4,r5,r6,r7,lr}
+	stmfd sp!,{r4,r5,r6,r7}
 __cp_begin:
 	ldr r0,[r0]
 	cmp r0,#0
-	blne __cp_cancel
+	bne __cp_cancel
 	mov r7,r1
 	mov r0,r2
 	mov r1,r3
 	ldmfd ip,{r2,r3,r4,r5,r6}
 	svc 0
 __cp_end:
-	ldmfd sp!,{r4,r5,r6,r7,lr}
+	ldmfd sp!,{r4,r5,r6,r7}
 	bx lr
 __cp_cancel:
-	ldmfd sp!,{r4,r5,r6,r7,lr}
+	ldmfd sp!,{r4,r5,r6,r7}
 	b __cancel