[ORC][LLJIT] Add a Pre-PlatformSetup-Setup function.

This function will be run prior to platform setup to provide LLJIT clients with
a chance to customize the LLJIT instance (e.g. install plugins) before the JIT
runtime is loaded.

The motivating use-case is debugger support: We want to install the debugger
plugin before the runtime is loaded (during platform setup) so that the runtime
itself can be debugged. A patch to do this will be committed shortly.