testsuite, Objective-C : Compatibility fixes.

testsuite, Objective-C : Compatibility fixes.

Changes to deal with warnings and/or errors seen when compiling the
tests with clang.

gcc/testsuite/ChangeLog:

	* objc-obj-c++-shared/TestsuiteObject.h: If the compiler
	supports objc_root_object attributes, then mark the testsuite
	object accordingly.
	* objc-obj-c++-shared/TestsuiteObject.m: Avoid direct access
	to isa, which is an error for modern Objective-C.
	* objc/execute/class-tests-1.h: Declare a string function we
	use locally (avoid pulling in the whole of string.h).