sha512-x86_64.pl: +16% optimization for Atom.

(and pending AVX2 changes).
(cherry picked from commit 504bbcf3cd236353d7e5ef66fc3bfbf5c7935791)
aesni-sha1-x86_64.pl: Atom-specific optimization.
(cherry picked from commit 4df2280b4f4747256a729116fef0f1c2af457522)