[Orc][LLJIT] Use JITLink on RISC-V

[Hahnfeld]

Summary

RuntimeDyld does not support RISC-V, so it makes sense to enable
JITLink by default. This also makes relocations work without support
for a large code model.

[lhames]

LGTM. Thanks @Hahnfeld!

llvm/lib/ExecutionEngine/Orc/LLJIT.cpp

LLVM_DEBUG({
       dbgs() << "Using explicitly specified ExecutorProcessControl instance "
              << EPC.get() << "\n";
     });
 
   // If the client didn't configure any linker options then auto-configure the
   // JIT linker.
   if (!CreateObjectLinkingLayer) {
     auto &TT = JTMB->getTargetTriple();
-    if (TT.isOSBinFormatMachO() &&
-        (TT.getArch() == Triple::aarch64 || TT.getArch() == Triple::x86_64)) {
+    if (TT.getArch() == Triple::riscv64 ||
+        (TT.isOSBinFormatMachO() &&
+         (TT.getArch() == Triple::aarch64 || TT.getArch() == Triple::x86_64))) {
 
       JTMB->setRelocationModel(Reloc::PIC_);
       JTMB->setCodeModel(CodeModel::Small);
       CreateObjectLinkingLayer =
           [](ExecutionSession &ES,
              const Triple &) -> Expected<std::unique_ptr<ObjectLayer>> {
         auto ObjLinkingLayer = std::make_unique<ObjectLinkingLayer>(ES);
         ObjLinkingLayer->addPlugin(std::make_unique<EHFrameRegistrationPlugin>(