runtime: Add vdso on freebsd/riscv64

runtime: Add vdso on freebsd/riscv64

Use rdtime to retrieve the timecounter, same as the FreeBSD libc.

Updates #53466

Change-Id: I48816e9100036f1ef483e4d3afcf10db0d3b85f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/443036
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Dmitri Goutnik <dgoutnik@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Yuval Pavel Zholkover <paulzhol@gmail.com>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>