internal/abi: define s390x ABI constants

Updates #40724

Change-Id: I9b6c56194b2cbc95c08441dfa1f779ed5efbadb8
Reviewed-on: https://go-review.googlesource.com/c/go/+/719461
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Vishwanatha HD <vishwanatha.hd@ibm.com>

src/internal/abi/abi_s390x.go

@@ -0,0 +1,19 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// 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 (
+	// See abi_generic.go.
+
+	// R2 - R9.
+	IntArgRegs = 8
+
+	// F0 - F15
+	FloatArgRegs = 16
+
+	EffectiveFloatRegSize = 8
+)