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.