arm: testsuite: fix issues relating to fp16 alternative testing

The v*_fp16_xN_1.c tests on Arm have been unstable since they were
added.  This is not a problem with the tests themselves, or even the
patches that were added, but with the testsuite infrastructure.  It
turned out that another set of dg- tests for fp16 were corrupting the
cached set of options used by the new tests, leading to running the
tests with incorrect flags.

So the primary goal of this patch is to fix the incorrect internal
caching of the options needed to enable fp16 alternative format on
Arm: the code was storing the result in the same variable that was
being used for neon_fp16 and this was leading to testsuite instability
for tests that were checking for neon with fp16.

But in cleaning this up I also noted that we weren't then applying the
flags correctly having detected what they were, so we also address
that.

I suspect there are still some further issues to address here, since
the framework does not correctly test that the multilibs and startup
code enable alternative format; but this is still an improvement over
what we had before.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp
	(check_effective_target_arm_fp16_alternative_ok_nocache): Use
	et_arm_fp16_alternative_flags to cache the result.  Improve test
	for FP16 availability.
	(add_options_for_arm_fp16_alternative): Use
	et_arm_fp16_alternative_flags.
	* g++.dg/ext/arm-fp16/arm-fp16-ops-3.C: Update dg-* flags.
	* 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.
	* gcc.target/arm/fp16-aapcs-3.c: Likewise.
	* gcc.target/arm/fp16-aapcs-4.c: Likewise.
	* gcc.target/arm/fp16-compile-alt-1.c: Likewise.
	* gcc.target/arm/fp16-compile-alt-10.c: Likewise.
	* gcc.target/arm/fp16-compile-alt-11.c: Likewise.
	* gcc.target/arm/fp16-compile-alt-12.c: Likewise.
	* gcc.target/arm/fp16-compile-alt-2.c: Likewise.
	* gcc.target/arm/fp16-compile-alt-3.c: Likewise.
	* gcc.target/arm/fp16-compile-alt-4.c: Likewise.
	* gcc.target/arm/fp16-compile-alt-5.c: Likewise.
	* gcc.target/arm/fp16-compile-alt-6.c: Likewise.
	* gcc.target/arm/fp16-compile-alt-7.c: Likewise.
	* gcc.target/arm/fp16-compile-alt-8.c: Likewise.
	* gcc.target/arm/fp16-compile-alt-9.c: Likewise.
	* gcc.target/arm/fp16-rounding-alt-1.c: Likewise.