Fix migrate from 32- to 64-bit processes

In some cases, it was possible to end up in a situation where the x64
reflective library hadn't been loaded by the time a user typed migrate.
If the target process was 64-bit, msfconsole would error out with a
NoMethodError and much sadness would ensue.

[See #2356]

I would love to have confirmation from @MosDefAssassin that this works.