testsuite/arm: Add arm_dsp_ok effective target and use it in arm/acle/dsp_arith.c

testsuite/arm: Add arm_dsp_ok effective target and use it in arm/acle/dsp_arith.c

gcc.target/arm/acle/dsp_arith.c uses DSP intrinsics, which arm_acle.h
defines only with __ARM_FEATURE_DSP, so make the test check for that
property rather than arm_qbit_ok.

However, the existing arm_dsp effective target only checks if DSP
features are supported with the current multilib rather than trying
-march and -mfloat-abi options. Thus we introduce a similar effective
target, arm_dsp_ok and associated dg-add-options.

This makes dsp_arith.c unsupported rather than failed when no option
combination is suitable, for instance when running the tests with
-mcpu=cortex-m3.

2021-03-19  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* doc/sourcebuild.texi (arm_dsp_ok, arm_dsp): Document.

	gcc/testsuite/
	* lib/target-supports.exp
	(check_effective_target_arm_dsp_ok_nocache)
	(check_effective_target_arm_dsp_ok, add_options_for_arm_dsp): New.
	* gcc.target/arm/acle/dsp_arith.c: Use arm_dsp_ok effective target
	and add arm_dsp options.