lavu/sha512: Fully unroll the transform function loops

crypto_bench SHA-512 results using an AMD Athlon X2 7750+, mingw32-w64 GCC 4.7.3 x86_64

Before:
lavu       SHA-512      size: 1048576  runs:   1024  time:   12.737 +- 0.147

After:
lavu       SHA-512      size: 1048576  runs:   1024  time:   11.670 +- 0.173

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
