Clean up alignment handling for SHA-512.

Clean up alignment handling for SHA-512.

This recommits r1.37 of sha512.c, however uses uint8_t * instead of void *
for the crypto_load_* functions and primarily uses const uint8_t * to track
input, only casting to const SHA_LONG64 * once we know that it is suitably
aligned. This prevents the compiler from implying alignment based on type.

Tested by tb@ and deraadt@ on platforms with gcc and strict alignment.

ok tb@