feat(riscv64,loong64): operand tail, float DATA and honest port classification

Assisted-by: GLM 5.3 Flash

testdata/verify/datainit_loong64.s

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+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Differential kernel for the loong64 DATA value forms the runtime's exp and
+// asm files use: floating-point initialisers stored as IEEE-754 bits and
+// string initialisers zero-padded within their declared width.
+
+#include "textflag.h"
+
+TEXT ·floatbits(SB), NOSPLIT, $0-8
+	MOVV $floats<>(SB), R12
+	MOVD 8(R12), F0
+	MOVD F0, ret+0(FP)
+	RET
+
+TEXT ·stringhead(SB), NOSPLIT, $0-8
+	MOVV $msg<>(SB), R12
+	MOVV (R12), R13
+	MOVV R13, ret+0(FP)
+	RET
+
+GLOBL floats<>(SB), RODATA, $16
+DATA floats<>+0(SB)/8, $0.0
+DATA floats<>+8(SB)/8, $0.5
+
+GLOBL msg<>(SB), RODATA, $20
+DATA msg<>+0(SB)/20, $"call frame too large"