testsuite: arm: require fp16 alternative hw for tests

Currently, it's assumed that arm_fp16_alternative_ok is enough to run
the tests, but arm_fp16_alternative_ok allows multilib override.  While
the test might link without trouble, execution will likely fail.
Add arm_fp16_alternative_hw to explicitly test that target can execute
fp16 instructions in alternative form.

Below tests are known to fail for
thumb/arch=armv7e-m+fp/cpu=cortex-m4/float-abi=hard/fpu=auto among other
targets.

gcc/ChangeLog:

	* doc/sourcebuild.texi (arm_fp16_alternative_hw): Document.
	(arm_neon_fp16_hw): Add reference to arm_neon_fp16_ok.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/arm-fp16/arm-fp16-ops-3.C: Require effective target
	arm_fp16_alternative_hw instead of arm_fp16_alternative_ok.
	* g++.dg/ext/arm-fp16/arm-fp16-ops-4.C: Likewise.
	* gcc.dg/torture/arm-fp16-int-convert-alt.c: Likewise.
	* gcc.dg/torture/arm-fp16-ops-3.c: Likewise.
	* gcc.dg/torture/arm-fp16-ops-4.c: Likewise.
	* lib/target-supports.exp (arm_fp16_alternative_hw): New proc.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>