sw64: define NR_SYSCALLS as generated __NR_syscalls

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56XYC



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With this definition, the asm/unistd.h can keep unchanged if the
number of syscalls has changed.

Signed-off-by: He Sheng <hesheng@wxiat.com>

Signed-off-by: Gu Zitao <guzitao@wxiat.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>