[Testsuite] Support ARMv8.1 ARM tests.

[Testsuite] Support ARMv8.1 ARM tests.

	testsuite/
	* lib/target-supports.exp (add_options_for_arm_v8_1a_neon): Update
	comment.  Use check_effective_target_arm_v8_1a_neon_ok to select
	the command line options.
	(check_effective_target_arm_v8_1a_neon_ok_nocache): Update initial
	test to allow ARM targets.  Select and record a working set of
	command line options.
	(check_effective_target_arm_v8_1a_neon_hw): Add tests for ARM
	targets.

	gcc/
	* doc/sourcebuild.texi (ARM-specific attributes): Add
	"arm_v8_1a_neon_ok" and "arm_v8_1a_neon_hw".

From-SVN: r231683