s390x: Add optimized chacha20

It adds vectorized ChaCha20 implementation based on libgcrypt
cipher/chacha20-s390x.S.  The final state register clearing is
omitted.

On a z15 it shows the following improvements (using formatted
bench-arc4random data):

GENERIC                                    MB/s
-----------------------------------------------
arc4random [single-thread]               198.92
arc4random_buf(16) [single-thread]       244.49
arc4random_buf(32) [single-thread]       282.73
arc4random_buf(48) [single-thread]       286.64
arc4random_buf(64) [single-thread]       320.06
arc4random_buf(80) [single-thread]       297.43
arc4random_buf(96) [single-thread]       310.96
arc4random_buf(112) [single-thread]      308.10
arc4random_buf(128) [single-thread]      309.90
-----------------------------------------------

VX.                                        MB/s
-----------------------------------------------
arc4random [single-thread]               430.26
arc4random_buf(16) [single-thread]       735.14
arc4random_buf(32) [single-thread]      1029.99
arc4random_buf(48) [single-thread]      1206.76
arc4random_buf(64) [single-thread]      1311.92
arc4random_buf(80) [single-thread]      1378.74
arc4random_buf(96) [single-thread]      1445.06
arc4random_buf(112) [single-thread]     1484.32
arc4random_buf(128) [single-thread]     1517.30
-----------------------------------------------

Checked on s390x-linux-gnu.