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

Assisted-by: GLM 5.3 Flash

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