feat(riscv): add prologue, epilogue and frame pseudo-register support

Assisted-by: Kimi K3

testdata/add_riscv64.s

@@ -0,0 +1,12 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+#include "textflag.h"
+
+// func add(a, b int64) int64
+TEXT ·add(SB), NOSPLIT, $0-24
+	LD a+0(FP), A0
+	LD b+8(FP), A1
+	ADD A0, A1, A0
+	SD A0, ret+16(FP)
+	RET