[Testsuite] Selectors and options directives for ARM VFP FP16 support.

[Testsuite] Selectors and options directives for ARM VFP FP16 support.

gcc/
	* doc/sourcebuild.texi (Effective-Target keywords): Add entries
	for arm_fp16_ok and arm_fp16_hw.
	(Add Options): Add entries for arm_fp16, arm_fp16_ieee and
	arm_fp16_alternative.

testsuite/
	* lib/target-supports.exp (add_options_for arm_fp16): Reword
	comment.
	(add_options_for_arm_fp16_ieee): New.
	(add_options_for_arm_fp16_alternative): New.
	(effective_target_arm_fp16_ok_nocache): Add to comment.  Fix a
	long-line.
	(effective_target_arm_fp16_hw): New.

From-SVN: r237847