Guess .dll on Windows when no data.

If you ignore the link error and use a DLL from another platform's
build, this gets things working on JVM on Windows.
Updates to CArray to convey type to REPR compose.

This will be needed for Moar to be able to handle CArray. Can maybe
get some further cleanups here in the future once everyone switches
over to the compose protocol. Didn't try on Parrot yet, but no JVM
regression on CArray tests with this patch.