powerpc64: Add optimized chacha20

It adds vectorized ChaCha20 implementation based on libgcrypt
cipher/chacha20-ppc.c.  It targets POWER8 and it is used on default
for LE.

On a POWER8 it shows the following improvements (using formatted
bench-arc4random data):

POWER8

GENERIC                                    MB/s
-----------------------------------------------
arc4random [single-thread]               138.77
arc4random_buf(16) [single-thread]       174.36
arc4random_buf(32) [single-thread]       228.11
arc4random_buf(48) [single-thread]       252.31
arc4random_buf(64) [single-thread]       270.11
arc4random_buf(80) [single-thread]       278.97
arc4random_buf(96) [single-thread]       287.78
arc4random_buf(112) [single-thread]      291.92
arc4random_buf(128) [single-thread]      295.25

POWER8                                     MB/s
-----------------------------------------------
arc4random [single-thread]               198.06
arc4random_buf(16) [single-thread]       278.79
arc4random_buf(32) [single-thread]       448.89
arc4random_buf(48) [single-thread]       551.09
arc4random_buf(64) [single-thread]       646.12
arc4random_buf(80) [single-thread]       698.04
arc4random_buf(96) [single-thread]       756.06
arc4random_buf(112) [single-thread]      784.12
arc4random_buf(128) [single-thread]      808.04
-----------------------------------------------

Checked on powerpc64-linux-gnu and powerpc64le-linux-gnu.
Reviewed-by: Paul E. Murphy <murphyp@linux.ibm.com>