Make the AAPCS64 function return tests more robust.

Make the AAPCS64 function return tests more robust.

gcc/testsuite

	* gcc.target/aarch64/aapcs64/abitest-2.h (saved_return_address): New
	global variable.
	(FUNC_VAL_CHECK): Update to call myfunc via the 'ret' instruction,
	instead of calling sequentially in the C code.
	* gcc.target/aarch64/aapcs64/abitest.S (LABEL_TEST_FUNC_RETURN): Store
	saved_return_address to the stack frame where LR register was stored.
	(saved_return_address): Declare weak.

From-SVN: r211954