Enable gdb-mi to set breakpoints when threads of debuggee are running

Enable gdb-mi to set breakpoints when threads of debuggee are running

* lisp/progmodes/gdb-mi.el (gdb): Call 'gud-def' with 5th arg
non-nil for those commands that can be invoked asynchronously.
(gdb-starting): Set 'gud-async-running' non-nil for async commands.
* lisp/progmodes/gud.el (gud-async-running): New defvar.
(gud-menu-map): Enable some commands only if 'gud-async-running' is
non-nil.
(gud-def): Accept another optional argument ASYNC-OK.  (Bug#65612)