Roll forward CL 171084886

171084886 had to be rolled back twice due to various open source build issues.
I'm trying again, now that I think I've addressed all the pertinent issues.

Original CL description:

Don't use dlsym to resolve symbols in the CPU JIT

Instead of resolving symbols via dlsym when JITting for the CPU backend, use a
registry based mechanism.  This lets us kill off the --export_dynamic hack that
we used to need for CustomCall on the CPU backend.

PiperOrigin-RevId: 173277862