Objective-C: Default flag_objc_sjlj_exceptions off for NeXT ABI >= 2.

Signed-off-by: Matt Jacobson <mhjacobson@me.com>

gcc/c-family/ChangeLog:

	* c-opts.c (c_common_post_options): Default to
	flag_objc_sjlj_exceptions = 1 only when flag_objc_abi < 2.

gcc/objc/ChangeLog:

	* objc-next-runtime-abi-02.c
	(objc_next_runtime_abi_02_init): Warn about and reset
	flag_objc_sjlj_exceptions regardless of flag_objc_exceptions.
	(next_runtime_02_initialize): Use a checking assert that
	flag_objc_sjlj_exceptions is off.