evp/e_chacha20_poly1305.c: further improve small-fragment TLS performance.

Improvement coefficients vary with TLS fragment length and platform, on
most Intel processors maximum improvement is ~50%, while on Ryzen - 80%.
The "secret" is new dedicated ChaCha20_128 code path and vectorized xor
helpers.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6638)