feat(verify): ABI checks on arm64, riscv64 and loong64

Assisted-by: GLM 5.3 Flash

testdata/verify/abi_riscv64.s

@@ -0,0 +1,21 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+#include "textflag.h"
+
+// func cleanAdd(a, b int64) int64
+// A well-behaved function that preserves the goroutine pointer.
+TEXT ·cleanAdd(SB), NOSPLIT, $0-24
+	MOV a+0(FP), X5
+	MOV b+8(FP), X6
+	ADD X5, X6, X5
+	MOV X5, ret+16(FP)
+	RET
+
+// func dirtyG(a int64) int64
+// Deliberately clobbers X27, the goroutine pointer (a serious ABI violation).
+TEXT ·dirtyG(SB), NOSPLIT, $0-16
+	MOV $0x5678, X27
+	MOV a+0(FP), X5
+	MOV X5, ret+8(FP)
+	RET