selftests: arm64: More comprehensively test the SVE ptrace interface

Currently the selftest for the SVE register set is not quite as thorough
as is desirable - it only validates that the value of a single Z register
is not modified by a partial write to a lower numbered Z register after
having previously been set through the FPSIMD regset.

Make this more thorough:
 - Test the ability to set vector lengths and enumerate those supported in
   the system.
 - Validate data in all Z and P registers, plus FPSR and FPCR.
 - Test reads via the FPSIMD regset after set via the SVE regset.

There's still some oversights, the main one being that due to the need to
generate a pattern in FFR and the fact that this rewrite is primarily
motivated by SME's streaming SVE which doesn't have FFR we don't currently
test FFR. Update the TODO to reflect those that occurred to me (and fix an
adjacent typo in there).
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210913125505.52619-8-broonie@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

tools/testing/selftests/arm64/fp/TODO

 - Test unsupported values in the ABIs.
-- More coverage for ptrace (eg, vector length conversions).
-- Coverage for signals.
-- Test PR_SVE_VL_INHERITY after a double fork.
+- More coverage for ptrace:
+ - Get/set of FFR.
+ - Ensure ptraced processes actually see the register state visible through
+   the ptrace interface.
+ - Big endian.
+- Test PR_SVE_VL_INHERIT after a double fork.