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.

gcc/testsuite/ChangeLog

+2020-04-08  Alexandre Oliva <oliva@adacore.com>
+
+	* gcc.target/arm/fp16-aapcs-3.c: Explicitly use the
+	-mfp16-format=alternative option.
+
 2020-04-08  H.J. Lu  <hongjiu.lu@intel.com>
 
 	PR target/94417

gcc/testsuite/gcc.target/arm/fp16-aapcs-3.c

 /* { dg-do compile }  */
 /* { dg-require-effective-target arm_hard_vfp_ok }  */
 /* { dg-require-effective-target arm_fp16_alternative_ok } */
-/* { dg-options "-O2" }  */
-/* { dg-add-options arm_fp16_alternative } */
+/* { dg-options "-O2 -mfp16-format=alternative" }  */
 
 /* Test __fp16 arguments and return value in registers (hard-float).  */