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