AArch64: Improve generic strlen

Improve performance by handling another 16 bytes before entering the loop.
Use ADDHN in the loop to avoid SHRN+FMOV when it terminates.  Change final
size computation to avoid increasing latency.  On Neoverse V1 performance
of the random strlen benchmark improves by 4.6%.

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