disable some aapcs/vfp*.c test if not arm_fp16_alternative_ok

disable some aapcs/vfp*.c test if not arm_fp16_alternative_ok

The tests use -mfp16-format=alternative, and so should not be run
if that option isn't supported.

for  gcc/testsuite/ChangeLog

	* lib/target-supports.exp
	(check_effective_target_arm_fp16_alternative_ok_nocache):
	Return zero for *-*-vxworks7r* targets.
	* gcc.target/arm/aapcs/vfp22.c: Require arm_fp16_alternative_ok.
	* gcc.target/arm/aapcs/vfp23.c: Likewise.
	* gcc.target/arm/aapcs/vfp24.c: Likewise.
	* gcc.target/arm/aapcs/vfp25.c: Likewise.