testsuite, Objective-C : Adjust gnu-api tests for Darwin.

testsuite, Objective-C : Adjust gnu-api tests for Darwin.

The GNU v2 API matches the next v2 API in most respects.

However, some of the tests depend on access to items that the
later NeXT headers consider to be 'internal implementation details'
and are not exposed (we arrange that with a DEFINE).

One test is skipped here because, although it works internally, the
number of objects returned is larger for the NeXT runtime in some
cases (where the headers have been updated to bring in more of the
system details).

gcc/testsuite/ChangeLog:

	* objc.dg/gnu-api-2-class-meta.m: Add a flag to cause NeXT
	headers to expose prototypes for messaging. Mark the root
	class if the objc_root_class attribute is available. Use
	char ** as the second arg to main.
	* objc.dg/gnu-api-2-class.m: Use dispatch prototype.
	* objc.dg/gnu-api-2-objc.m: Skip on NeXT because of extra
	prototypes pulled in by headers.