[lli] Add option -lljit-platform=Inactive to disable platform support explicitly

This option tells LLJIT to disable platform support explicitly: JITDylibs aren't scanned for special init/deinit symbols and no runtime API interposes are injected.
It's useful in two cases: for platforms that don't have such requirements and platforms for which we have no explicit support yet and that don't work well with the generic IR platform.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D99416