Partial revert of 5f2e704.

Specifically:

    Finally, for x86_64 Parrot running a 32 bit application (pfs_dispatch.cc), we
    change POINTER to remove the leading 32 bits. There was an issue (which causes
    changed to a 64 bit high address. e.g.

    0xffde0000

    becomes

    0xffffffffffde0000

    So we 0 the leading 32 bits to fix that.

    Fixes #511.

This was untrue. I had done testing on the wrong machine and was confused by
some phantom bug.

I'm still not sure what is causing #511.