stubs: split semihosting_get_target from system only stubs

As we are about to have a common syscalls.c for gdbstub we need to
stub out one semihosting helper function for all targets.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230307170405.796822-1-alex.bennee@linaro.org>

stubs/semihost-all.c

@@ -0,0 +1,17 @@
+/*
+ * Semihosting Stubs for all targets
+ *
+ * Copyright (c) 2023 Linaro Ltd
+ *
+ * Stubs for all targets that don't actually do semihosting.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "qemu/osdep.h"
+#include "semihosting/semihost.h"
+
+SemihostingTarget semihosting_get_target(void)
+{
+    return SEMIHOSTING_TARGET_AUTO;
+}