[lli] Mimic default LLJIT logic with explicit --jit-linker option (#76964)

Forcing the JITLinker to the default configuration shouldn't affect the
configuration of the JIT. Since it's implemented by setting the
ObjectLinkingLayerCreator, it did have side-effects though. In
particular, we skipped the Code- and RelocationModel setting in
`LLJITBuilderState::prepareForConstruction()`.