SF bug #442520: test_struct fails on SPARC.

The ob_sval member of a string object isn't necessarily aligned to better
than a native long, so the new "q" and "Q" struct codes can't get away w/
casting tricks on platforms where LONG_LONG requires stricter-than-long
alignment.  After I thought of a few elaborate workarounds, Guido bashed
me over the head with the obvious memcpy approach, herewith implemented.