add missing fp16 options

add missing fp16 options

dg-require-effective-target arm_fp16_alternative_ok may pass even when
arm_fp16_ok doesn't, and the latter's failure inhibits dg-add-options
arm_fp16_alternative.  Requiring arm_fp16_ok would disable the test,
but if we just pass it the -mfp16-format=alternative option, it passes
even without arm_fp16_ok.  Sibling test fp16-aapcs-4.c underwent a
similar change, so I'm proposing the explicit option to fp16-aapcs-3.c
as well.

for  gcc/testsuite/ChangeLog

	* gcc.target/arm/fp16-aapcs-3.c: Explicitly use the
	-mfp16-format=alternative option.