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>