Avoid multiple calls to memcpy (src/backend/access/index/genam.c)

Hi.

In the functions *systable_beginscan* and *systable_beginscan_ordered*,
is possible a small optimization.
The array *idxkey* can be constructed in one go with a single call to
mempcy.
The excess might not make much of a difference, but I think it's worth the
effort.

patch attached.

best regards,
Ranier Vilela