[Orc] Let LLJIT default to JITLink for ELF-based ARM targets (#77313)

The JITLink AArch32 backend reached feature-parity with RuntimeDyld
on ELF-based systems. This patch changes the default JIT-linker in Orc's
LLJIT for these platforms.

This allows us to run clang-repl with JITLink on ARM and use all the
features we had with RuntimeDyld before. All existing tests for
clang-repl are passing.