[dev.cc] runtime/cgo: change PC to R15 in asm_arm.s

R15 is the real register. PC is a pseudo-register that we are making
illegal in this context as part of the grand assembly unification.

Change-Id: Ie0ea38ce7ef4d2cf4fcbe23b851a570fd312ce8d
Reviewed-on: https://go-review.googlesource.com/4966Reviewed-by: Minux Ma <minux@golang.org>

src/runtime/cgo/asm_arm.s

@@ -19,6 +19,6 @@ TEXT crosscall2(SB),NOSPLIT,$-4
 	 */
 	MOVM.WP	[R0, R1, R2, R4, R5, R6, R7, R8, R9, g, R11, R12, R14], (R13)
 	BL	runtime·load_g(SB)
-	MOVW	PC, R14
-	MOVW	0(R13), PC
-	MOVM.IAW	(R13), [R0, R1, R2, R4, R5, R6, R7, R8, R9, g, R11, R12, PC]
+	MOVW	R15, R14 // R15 is PC.
+	MOVW	0(R13), R15
+	MOVM.IAW	(R13), [R0, R1, R2, R4, R5, R6, R7, R8, R9, g, R11, R12, R15]