Add a 'single lane' SIMD implemementation of the single byte count
function for use on machines that support the vector facility. This
allows up to 16 bytes to be counted per loop iteration.

We can probably improve performance further by adding more 'lanes'
(i.e. counting more bytes in parallel) however this will increase
the complexity of the function so I'm not sure it is worth doing
yet.

name                old speed      new speed       delta
pkg:strings goos:linux goarch:s390x
CountByte/10         789MB/s ± 0%   1131MB/s ± 0%    +43.44%  (p=0.000 n=9+9)
CountByte/32         936MB/s ± 0%   3236MB/s ± 0%   +245.87%  (p=0.000 n=8+9)
CountByte/4096      1.06GB/s ± 0%  21.26GB/s ± 0%  +1907.07%  (p=0.000 n=10+10)
CountByte/4194304   1.06GB/s ± 0%  20.54GB/s ± 0%  +1838.50%  (p=0.000 n=10+10)
CountByte/67108864  1.06GB/s ± 0%  18.31GB/s ± 0%  +1629.51%  (p=0.000 n=10+10)
pkg:bytes goos:linux goarch:s390x
CountSingle/10       800MB/s ± 0%    986MB/s ± 0%    +23.21%  (p=0.000 n=9+10)
CountSingle/32       925MB/s ± 0%   2744MB/s ± 0%   +196.55%  (p=0.000 n=9+10)
CountSingle/4K      1.26GB/s ± 0%  19.44GB/s ± 0%  +1445.59%  (p=0.000 n=10+10)
CountSingle/4M      1.26GB/s ± 0%  20.28GB/s ± 0%  +1510.26%  (p=0.000 n=8+10)
CountSingle/64M     1.23GB/s ± 0%  17.78GB/s ± 0%  +1350.67%  (p=0.000 n=9+10)

Change-Id: I230d57905db92a8fdfc50b1d5be338941ae3a7a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/199979
Run-TryBot: Michael Munday <mike.munday@ibm.com>
Reviewed-by: Keith Randall <khr@golang.org>