Clean up SHA-512 input handling and round macros.

Clean up SHA-512 input handling and round macros.

Avoid reach around and initialisation outside of the macro, cleaning up
the call sites to remove the initialisation. Use a T2 variable to more
closely follow the documented algorithm and remove the gorgeous compound
statement X = Y += A + B + C.

There is no change to the clang generated assembly on aarch64.

ok tb@