alpha: replace NR_SYSCALLS by NR_syscalls

Reference to other arch likes x86_64 or arm64 to do this replacement.
To solve compile error when using NR_syscalls in kernel[1].

[1] https://lore.kernel.org/all/202203270449.WBYQF9X3-lkp@intel.com/



Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
Signed-off-by: Matt Turner <mattst88@gmail.com>