Support -Wabi warning about backward compatibility.

gcc/c-family/
	* c.opt (Wabi=, fabi-compat-version): New.
	* c-opts.c (c_common_handle_option): Handle -Wabi=.
	(c_common_post_options): Handle flag_abi_compat_version default.
	Disallow -fabi-compat-version=1.
	* c-common.h (abi_version_crosses): New.
gcc/cp/
	* call.c (convert_arg_to_ellipsis): Use abi_version_crosses.
	* cvt.c (type_promotes_to): Likewise.
	* mangle.c (write_type, write_expression): Likewise.
	(write_name, write_template_arg): Likewise.
	(mangle_decl): Make alias based on flag_abi_compat_version.
	Emit -Wabi warning here.
	(finish_mangling_internal): Not here.  Drop warn parm.
	(finish_mangling_get_identifier, finish_mangling): Adjust.
	(mangle_type_string, mangle_special_for_type): Adjust.
	(mangle_ctor_vtbl_for_type, mangle_thunk): Adjust.
	(mangle_guard_variable, mangle_tls_init_fn): Adjust.
	(mangle_tls_wrapper_fn, mangle_ref_init_variable): Adjust.

From-SVN: r211594