runtime: init plan9 hashkey by time

Maphash requires non-zero integer for initial hashkey

Fixes #45090

Change-Id: Ie567f648c19e81cddc8e72a1c64809fbf52df188
Reviewed-on: https://go-review.googlesource.com/c/go/+/303969


Trust: Meng Zhuo <mzh@golangcn.org>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Richard Miller <millerresearch@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>