ripemd: move ripemd{256, 320} into separate functions

This allows the removal of a few branches.

Before:
lavu       RIPEMD-160   size: 1048576  runs:   1024  time:    7.052 +- 0.010

After:
lavu       RIPEMD-160   size: 1048576  runs:   1024  time:    6.865 +- 0.015
Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>