[JITLink][RISCV] Expose relaxation pass publicly

[jobnoorman]

Summary

This is useful for contexts where shouldAddDefaultTargetPasses returns
false but that still want to perform relaxation.

[StephenFan]

LGTM.

llvm/include/llvm/ExecutionEngine/JITLink/ELF_riscv.h

 /// outlives the graph.
 Expected<std::unique_ptr<LinkGraph>>
 createLinkGraphFromELFObject_riscv(MemoryBufferRef ObjectBuffer);
 
 /// jit-link the given object buffer, which must be a ELF riscv object file.
 void link_ELF_riscv(std::unique_ptr<LinkGraph> G,
                     std::unique_ptr<JITLinkContext> Ctx);
 
+/// Returns a pass that performs linker relaxation. Should be added to
+/// PreFixupPasses.
+LinkGraphPassFunction createRelaxationPass_ELF_riscv();
+
 } // end namespace jitlink
 } // end namespace llvm
 
 #endif // LLVM_EXECUTIONENGINE_JITLINK_ELF_RISCV_H

llvm/lib/ExecutionEngine/JITLink/ELF_riscv.cpp

if (Ctx->shouldAddDefaultTargetPasses(TT)) {
     Config.PreFixupPasses.push_back(relax);
   }
   if (auto Err = Ctx->modifyPassConfig(*G, Config))
     return Ctx->notifyFailed(std::move(Err));
 
   ELFJITLinker_riscv::link(std::move(Ctx), std::move(G), std::move(Config));
 }
 
+LinkGraphPassFunction createRelaxationPass_ELF_riscv() { return relax; }
+
 } // namespace jitlink
 } // namespace llvm