avoid unaligned access errors, e.g., on sparc

avoid unaligned access errors, e.g., on sparc

* lib/sha512.c (sha512_conclude_ctx): Use set_uint64 rather than
direct access through a possibly-unaligned uint64* pointer.
* lib/sha256.c (sha256_conclude_ctx): Use set_uint32 rather than
direct access through a possibly-unaligned uint32* pointer.
Prompted by this patch from Tom "spot" Callaway:
http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/13638