selftests/powerpc: Fix fpu_signal failures

My recent commit e5d00aaa ("selftests/powerpc: Check all FPRs in
fpu_preempt") inadvertently broke the fpu_signal test.

It needs to take into account that fpu_preempt now loads 32 FPRs, so
enlarge darray.

Also use the newly added randomise_darray() to properly randomise darray.

Finally the checking done in signal_fpu_sig() needs to skip checking
f30/f31, because they are used as scratch registers in check_all_fprs(),
called by preempt_fpu(), and so could hold other values when the signal
is taken.

Fixes: e5d00aaa ("selftests/powerpc: Check all FPRs in fpu_preempt")
Reported-by: Spoorthy <spoorthy@linux.ibm.com>
Depends-on: 2ba107f6 ("selftests/powerpc: Generate better bit patterns for FPU tests")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240301101035.1230024-1-mpe@ellerman.id.au