[ORC][LLJIT] Expose ORCPlatformSupport to aid custom native Platform setup.

LLJIT users may want to customize their native platform setup beyond what the
ExecutorNativePlatform helper permits. In this case LLJIT users can construct
the platform instance manually, and then add an ORCPlatformSupport instance
to forward initialize/deinitialize operations to the ORC runtime.