runtime: use riscv64 RDTIME instruction
runtime: use riscv64 RDTIME instruction

Use the actual RDTIME instruction, rather than a WORD.
Generated code is the same.

Change-Id: I6f6f5a1836eae2d05af34d4a22db2ede4fdcb458
Reviewed-on: https://go-review.googlesource.com/c/go/+/231997
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,golang/go,golang/go,nawawi/go