AArch64: Fix and improve SVE pow(f) special cases

AArch64: Fix and improve SVE pow(f) special cases

powf:

Update scalar special case function to best use new interface.

pow:

Make specialcase NOINLINE to prevent str/ldr leaking in fast path.
Remove depency in sv_call2, as new callback impl is not a
performance gain.
Replace with vectorised specialcase since structure of scalar
routine is fairly simple.

Throughput gain of about 5-10% on V1 for large values and 25% for subnormal `x`.

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>