sw64: vdso: fix time calculation

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56P0Z



--------------------------------

Switch type of wall to monotonic time to u64 so we can get the
correct time.

Signed-off-by: Mao Minkai <maominkai@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>