internal/abi, internal/buildcfg: always enable register ABI on riscv64

internal/abi, internal/buildcfg: always enable register ABI on riscv64

In Go 1.19, we develop register ABI for riscv64, enabled by default as a
GOEXPERIMENT. We can turn it on all the time in Go 1.20.

Change-Id: Ie8e2ac8b8bd3ebddb0dc6d58a5599547fb440e03
Reviewed-on: https://go-review.googlesource.com/c/go/+/443555
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>