Separate HAL driver registration with the threading (#7184)

* Separate HAL driver registration with the threading

Have synced drivers for {Dylib, VMVX}, and the global flags of
`IREE_HAL_DRIVER_{DYLIB, VMVX}_SYNC`, so the HAL driver registration can be
done with threadless configuration.

Enable iree/runtime libraries to be built in threadless configuration.

Clean up the usage of IREE_ENABLE_THREADING flag in iree/hal and iree/samples

Part of the cleanup for https://github.com/google/iree/issues/4298

* Make `dylib-sync` driver embedded library compatible only

This allows iree/runtime can really built in the bare-metal config
without the dynamic_library dependency, and the `-ldl` linker option.

This also removes the driver dependency in iree/samples/{dynamic_shapes,
variable_and_state} examples.