runtime: use vDSO clock_gettime on linux/riscv64

runtime: use vDSO clock_gettime on linux/riscv64

Speed up nanotime1 and walltime on riscv64 with vDSO, just like the
other vDSO-enabled targets.

name          old time/op  new time/op  delta
Now           1.53µs ± 1%  1.05µs ± 3%  -31.74%  (p=0.000 n=10+10)
NowUnixNano   1.54µs ± 0%  1.05µs ± 2%  -31.91%  (p=0.000 n=9+10)
NowUnixMilli  1.56µs ± 1%  1.06µs ± 1%  -31.73%  (p=0.000 n=10+10)
NowUnixMicro  1.56µs ± 1%  1.05µs ± 1%  -32.80%  (p=0.000 n=9+10)

Change-Id: I69b3fe3cc57685a826c53b366c0992e8048399bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/328509
Trust: Meng Zhuo <mzh@golangcn.org>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>