[dev.cc] misc/cgo/test: fix PC reference in arm assembler

Use R15.
May fix build.

Change-Id: Ia25b0936c5aab2a427f8e6531688c3e537fbfdd0
Reviewed-on: https://go-review.googlesource.com/5070Reviewed-by: Russ Cox <rsc@golang.org>

misc/cgo/test/issue9400/asm_arm.s

 #include "textflag.h"
 
 TEXT cas<>(SB),NOSPLIT,$0
-	MOVW	$0xffff0fc0, PC
+	MOVW	$0xffff0fc0, R15 // R15 is PC
 
 TEXT ·RewindAndSetgid(SB),NOSPLIT,$-4-0
 	// Save link register