Unroll loop to improve perfromance back to 1.8 level.
name              old time/op    new time/op    delta
EncodeToString-6    63.0µs ± 3%    51.7µs ± 2%  -17.94%  (p=0.000 n=10+10)

name              old speed      new speed      delta
EncodeToString-6   130MB/s ± 3%   159MB/s ± 2%  +21.83%  (p=0.000 n=10+10)

Vs 1.8:
EncodeToString-6    54.9µs ± 2%    51.7µs ± 2%   -5.95%  (p=0.000 n=10+10)

name              old speed      new speed      delta
EncodeToString-6   149MB/s ± 2%   159MB/s ± 2%   +6.32%  (p=0.000 n=10+10)

Fixes #21262

Change-Id: I41bf7e1f61041781386d16d573bffe1a7173c0c3
Reviewed-on: https://go-review.googlesource.com/52510
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>