Make SJLJ exceptions more efficient

Make SJLJ exceptions more efficient

This changes the SJLJ exception handling code to be a bit more
efficient, by using rvalue references and move assignment when
possible.

Tested by the buildbot.

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

	* event-top.c (gdb_rl_callback_read_char_wrapper_noexcept)
	(gdb_rl_callback_handler): Use std::move.
	* common/common-exceptions.h (struct gdb_exception): Add move
	assignment operator.
	(throw_exception_sjlj): Change "exception" to const reference.
	* common/common-exceptions.c (exceptions_state_mc_catch): Update.
	(throw_exception_sjlj): Change "exception" to const reference.