internal/abi, internal/buildcfg: always enable register ABI on PPC64

In last cycle we developed register ABI for ARM64, enabled by
default as a GOEXPERIMENT. This cycle we turn it on all the time.
Later CLs will clean up fallback code.

Change-Id: Idac4dcff634791cbc3d30988052ecd742b55ab8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/394214
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

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
+//go:build !goexperiment.regabiargs && !amd64 && !arm64 && !ppc64 && !ppc64le
 
 package abi
 

src/internal/abi/abi_ppc64x.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 && (ppc64 || ppc64le)
+//go:build ppc64 || ppc64le
 
 package abi
 

src/internal/buildcfg/exp.go

@@ -62,10 +62,8 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 	// always on.
 	var regabiSupported, regabiAlwaysOn bool
 	switch goarch {
-	case "amd64", "arm64":
+	case "amd64", "arm64", "ppc64le", "ppc64":
 		regabiAlwaysOn = true
-		fallthrough
-	case "ppc64le", "ppc64":
 		regabiSupported = true
 	}