Revert "[ORC][LLJIT] Use JITLink by default on ELF/x86-64."

This reverts commit 85c649bc02.

Has been causing test failures in OrcV2Examples for the past few
days.

llvm/lib/ExecutionEngine/Orc/LLJIT.cpp

@@ -750,7 +750,7 @@ Error LLJITBuilderState::prepareForConstruction() {
       UseJITLink = !TT.isOSBinFormatCOFF();
       break;
     case Triple::x86_64:
-      UseJITLink = !TT.isOSBinFormatCOFF();
+      UseJITLink = TT.isOSBinFormatMachO();
       break;
     default:
       break;