Fix memory leak in exception code

Fix memory leak in exception code

PR gdb/24475 concerns a memory leak coming from gdb's exception
handling code.

The leak occurs because throw_exception_sjlj does not arrange to
destroy the exception object it is passed.  However, because
gdb_exception has a destructor, it's undefined to longjmp in this
situation.

This patch fixes the problem by avoiding the need to run any
destructors in gdb_rl_callback_handler, by making the gdb_exception
"static".

gdb/ChangeLog
2019-04-25  Tom Tromey  <tromey@adacore.com>

	PR gdb/24475:
	* event-top.c (gdb_rl_callback_handler): Make "gdb_rl_expt"
	static.