internal/chacha20: improve performance for ppc64le

Add asm implementation for chacha20 using vector instructions on ppc64le.
Below, the difference using the new code:

name               old speed     new speed      delta
ChaCha20/32-16     167MB/s ± 0%   129MB/s ± 0%   -22.60%  (p=0.008 n=5+5)
ChaCha20/63-16     308MB/s ± 0%   249MB/s ± 0%   -19.00%  (p=0.008 n=5+5)
ChaCha20/64-16     357MB/s ± 0%   251MB/s ± 0%   -29.57%  (p=0.008 n=5+5)
ChaCha20/256-16    398MB/s ± 0%  1199MB/s ± 0%  +201.20%  (p=0.008 n=5+5)
ChaCha20/1024-16   413MB/s ± 0%  1398MB/s ± 0%  +238.67%  (p=0.008 n=5+5)
ChaCha20/1350-16   395MB/s ± 0%  1189MB/s ± 0%  +200.71%  (p=0.008 n=5+5)
ChaCha20/65536-16  420MB/s ± 0%  1489MB/s ± 0%  +254.10%  (p=0.008 n=5+5)

Small sizes are slower due the fact that it always calculates using
256 bytes of key stream.

This change was originally from Paulo Flabiano Smorigo <pfsmorigo@linux.vnet.ibm.com>
and started as CL 108999 (https://go-review.googlesource.com/c/crypto/+/108999).

Fixes golang/go#25051

Change-Id: Ie510494249b227379e23d993467256b3d4088035
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/172177
Run-TryBot: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>

Author: ceseo