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 <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: wdvxdr1123

src/internal/abi/abi_generic.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !goexperiment.regabiargs && !amd64 && !arm64 && !ppc64 && !ppc64le
+//go:build !goexperiment.regabiargs && !amd64 && !arm64 && !ppc64 && !ppc64le && !riscv64
 
 package abi
 

src/internal/abi/abi_riscv64.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build goexperiment.regabiargs
-
 package abi
 
 const (

src/internal/buildcfg/exp.go

@@ -62,11 +62,9 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 	// always on.
 	var regabiSupported, regabiAlwaysOn bool
 	switch goarch {
-	case "amd64", "arm64", "ppc64le", "ppc64":
+	case "amd64", "arm64", "ppc64le", "ppc64", "riscv64":
 		regabiAlwaysOn = true
 		regabiSupported = true
-	case "riscv64":
-		regabiSupported = true
 	}
 
 	baseline := goexperiment.Flags{