semihosting: Split out semihost_sys_open

Split out the non-ARM specific portions of SYS_OPEN to a
reusable function.  This handles gdb and host file i/o.

Add helpers to validate the length of the filename string.
Prepare for usage by other semihosting by allowing the
filename length parameter to be 0, and calling strlen.

Reviewed-by: Luc Michel <lmichel@kalray.eu>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

include/semihosting/syscalls.h

@@ -0,0 +1,25 @@
+/*
+ * Syscall implementations for semihosting.
+ *
+ * Copyright (c) 2022 Linaro
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#ifndef SEMIHOSTING_SYSCALLS_H
+#define SEMIHOSTING_SYSCALLS_H
+
+/*
+ * Argument loading from the guest is performed by the caller;
+ * results are returned via the 'complete' callback.
+ *
+ * String operands are in address/len pairs.  The len argument may be 0
+ * (when the semihosting abi does not already provide the length),
+ * or non-zero (where it should include the terminating zero).
+ */
+
+void semihost_sys_open(CPUState *cs, gdb_syscall_complete_cb complete,
+                       target_ulong fname, target_ulong fname_len,
+                       int gdb_flags, int mode);
+
+#endif /* SEMIHOSTING_SYSCALLS_H */