feat(asm): encode indirect JMP and CALL on all four architectures

Assisted-by: GLM 5.3 Flash

testdata/verify/indirect_riscv64.s

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+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Indirect control flow: JMP (X5) lowers to JALR X0, 0(X5), the trampoline
+// form JALR rd, offset(rs1) encodes with the destination first, and a linking
+// JALR through X1 is the toolchain's only indirect call.
+
+#include "textflag.h"
+
+// func f()
+TEXT ·leaf(SB), NOSPLIT, $0-0
+	JMP (X5)
+	JALR X0, 0(X6)
+	RET
+
+// func g()
+TEXT ·calls(SB), NOSPLIT, $0-0
+	JALR X1, 0(X8)
+	RET