[testsuite,arm] target-supports.exp: Add arm_fp_dp_ok effective-target

[testsuite,arm] target-supports.exp: Add arm_fp_dp_ok effective-target

Some tests require double-precision support, but the existing
arm_fp_ok effective-target only checks if hardware floating-point is
available, not what level. So this patch adds a new arm_fp_dp_ok
effective-target to check that double-precision is supported.

2020-03-24  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* doc/sourcebuild.texi (ARM-specific attributes): Add
	arm_fp_dp_ok.
	(Features for dg-add-options): Add arm_fp_dp.

	gcc/testsuite/
	* lib/target-supports.exp
	(check_effective_target_arm_fp_dp_ok_nocache): New.
	(check_effective_target_arm_fp_dp_ok): New.
	(add_options_for_arm_fp_dp): New.