Remove .arch directives from spinlock.S

This patch removes .arch "arm8.1-a" and "armv8-a"
directives which overwrite ASFLAGS_aarch64 option based
on ARM_ARCH_MINOR passed to Makefile and cause
translation errors like
"selected processor does not support `bti jc'"
for armv8.5-a targets when BTI support is enabled.

Change-Id: Idca5b66ed1e5d86e2188b0c0f16c3819990957c4
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>

lib/locks/exclusive/aarch64/spinlock.S

 /*
- * Copyright (c) 2013-2017, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2013-2019, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -39,8 +39,6 @@
 
 #if USE_CAS
 
-	.arch	armv8.1-a
-
 /*
  * Acquire lock using Compare and Swap instruction.
  *
@@ -60,8 +58,6 @@ func spin_lock
 	ret
 endfunc spin_lock
 
-	.arch	armv8-a
-
 #else /* !USE_CAS */
 
 /*