[RISCV] Fix 64-bit data layout mismatch between backend and target description

[asb]

This is the correct string, as used and tested in the yet-to-be merged patches. I forgot to move this fix from the relevant RV64I patch. Thanks for noticing this.

llvm/trunk/lib/Target/RISCV/RISCVTargetMachine.cpp

 
 extern "C" void LLVMInitializeRISCVTarget() {
   RegisterTargetMachine<RISCVTargetMachine> X(getTheRISCV32Target());
   RegisterTargetMachine<RISCVTargetMachine> Y(getTheRISCV64Target());
 }
 
 static std::string computeDataLayout(const Triple &TT) {
   if (TT.isArch64Bit()) {
-    return "e-m:e-i64:64-n32:64-S128";
+    return "e-m:e-p:64:64-i64:64-i128:128-n64-S128";
   } else {
     assert(TT.isArch32Bit() && "only RV32 and RV64 are currently supported");
     return "e-m:e-p:32:32-i64:64-n32-S128";
   }
 }
 
 static Reloc::Model getEffectiveRelocModel(const Triple &TT,
                                            Optional<Reloc::Model> RM) {