(perl #127743) support for >= 4GB data from hooks

(perl #127743) support for >= 4GB data from hooks

Though we use this mechanism for >= 2GB.

This emits an SX_LOBJECT op and writes the string length as a 64-bit
value if STORABLE_attach() returns more than 2GB of data.

The boundary is set at 2GB since older versions of Storable handle
sizes between 2GB and 4GB badly, resulting in a memory overwrite.  By
using the alternate op codes an older Storable will reject the data
instead.

We still accept such data that might have been written by an older
Storable.