[ORC][ORC_RT] Integrate ORC platforms with LLJIT and lli

This change enables integrating orc::LLJIT with the ORCv2
platforms (MachOPlatform and ELFNixPlatform) and the compiler-rt orc
runtime. Changes include:

- Adding SPS wrapper functions for the orc runtime's dlfcn emulation
  functions, allowing initialization and deinitialization to be invoked
  by LLJIT.

- Changing the LLJIT code generation default to add UseInitArray so
  that .init_array constructors are generated for ELF platforms.

- Integrating the ORCv2 Platforms into lli, and adding a
  PlatformSupport implementation to the LLJIT instance used by lli which
  implements initialization and deinitialization by calling the new
  wrapper functions in the runtime.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D126492