PyGC_Head: Use "long double" instead of "double" as the worst-case

PyGC_Head:  Use "long double" instead of "double" as the worst-case
alignment gimmick.  David Abrahams notes that the standard "long double"
actually requires stricter alignment than "double" on some Tru64 box.
On my box and yours <wink>, it's the same, so no harm done on most
boxes.