aarch64: Optimise AdvSIMD log2f

aarch64: Optimise AdvSIMD log2f

Optimise AdvSIMD log2f by vectorising the special case.
Use scaling technique on subnormal values, then check for inf and
nan values.
The scaling technique used will sqrt the input then multiply the
output by 2 because:
log(sqrt(x)) = 1/2 log(x), so log(x) = 2log(sqrt(x))

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>