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

Assisted-by: GLM 5.3 Flash

testdata/verify/indirect_amd64.s

@@ -0,0 +1,18 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Indirect control flow: JMP/CALL through a register or memory, byte-compared
+// against go tool asm.  A CALL in the body also exercises the toolchain's
+// forced base-pointer frame on a frameless function.
+
+#include "textflag.h"
+
+// func f()
+TEXT ·f(SB), NOSPLIT, $0
+	JMP AX
+	CALL AX
+	JMP (BX)
+	CALL (BX)
+	JMP 8(BX)
+	JMP R8
+	RET

testdata/verify/indirect_arm64.s

@@ -0,0 +1,14 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Indirect control flow: JMP (R0) and CALL (R0) lower to BR/BLR, the only
+// indirect-branch spellings the toolchain accepts (the raw BR/BLR mnemonics
+// stay a gasm superset).
+
+#include "textflag.h"
+
+// func f()
+TEXT ·f(SB), NOSPLIT, $0
+	JMP (R0)
+	CALL (R0)
+	RET

testdata/verify/indirect_loong64.s

@@ -0,0 +1,14 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Indirect control flow: JMP (R4) and JAL (R5) are the toolchain's spellings
+// for jirl; the raw JIRL instruction is deliberately absent, because the Go
+// loong64 assembler deletes it and a parity kernel could not hold it.
+
+#include "textflag.h"
+
+// func f()
+TEXT ·f(SB), NOSPLIT, $0-0
+	JMP (R4)
+	JAL (R5)
+	RET

testdata/verify/indirect_riscv64.s

@@ -0,0 +1,19 @@
+// 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