internal/abi: define loong64 regABI constants

Update #40724

Co-authored-by: Xiaolin Zhao <zhaoxiaolin@loongson.cn>
Change-Id: Id580d9e22a562adee2ae02a467ac38a54949e737
Reviewed-on: https://go-review.googlesource.com/c/go/+/521778
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: David Chase <drchase@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David Chase <drchase@google.com>

src/internal/abi/abi_loong64.go

@@ -0,0 +1,19 @@
+// Copyright 2023 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.
+
+	// R4 - R19
+	IntArgRegs = 16
+
+	// F0 - F15
+	FloatArgRegs = 16
+
+	EffectiveFloatRegSize = 8
+)