runtime: simplify the implementation of walltime and nanotime1 on loong64

runtime: simplify the implementation of walltime and nanotime1 on loong64

goos: linux
goarch: loong64
pkg: time
cpu: Loongson-3A6000 @ 2500.00MHz
              |   old.txt   |   new.txt                          |
              |   sec/op    |   sec/op      vs base              |
Now             34.82n ± 0%   34.24n ± 0%  -1.67% (p=0.000 n=10)
NowUnixNano     35.44n ± 0%   35.82n ± 0%  +1.07% (p=0.000 n=10)
NowUnixMilli    36.05n ± 0%   34.24n ± 0%  -5.02% (p=0.000 n=10)
NowUnixMicro    36.83n ± 0%   34.83n ± 0%  -5.43% (p=0.000 n=10)
geomean         35.78n        34.78n       -2.80%

Change-Id: I6ab143b61dbf94c91df2e9dfb80ec9b908219825
Reviewed-on: https://go-review.googlesource.com/c/go/+/741380
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: Meidan Li <limeidan@loongson.cn>