Move exception_none to common code, and use it

Move exception_none to common code, and use it

gdb/ChangeLog:
2015-02-27  Pedro Alves  <palves@redhat.com>

	* common/common-exceptions.h (exception_none): Declare.
	* common/common-exceptions.c (exception_none): Moved from
	exceptions.c.
	(exceptions_state_mc_init): Use exception_none.
	* exceptions.c (exception_none): Move to
	common/common-exceptions.c.
	* exceptions.h (exception_none): Move to
	common/common-exceptions.h.