[ORC] Add initial support for aarch64 in ELFNixPlatform

[sunho]

Summary

This patch adds the aarch64 support in ELFNixPlatform. These are few simple changes, but it allows us to use the orc runtime in ELF/AARCH64 backend. I observed that it succesfully run the static initializers of libc++ iostream implementation so that "cout << Hello world" testcase starts to work.

[lhames]

Does that mean that we were already building the ORC runtime for ELF/arm64? I think that means that we missed some gating in the build system: we should only have been building it for arm64 on Darwin.

It comes in handy here though. LGTM. :)

llvm/lib/ExecutionEngine/Orc/ELFNixPlatform.cpp

 //===------ ELFNixPlatform.cpp - Utilities for executing MachO in Orc -----===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ExecutionEngine/Orc/ELFNixPlatform.h"
 
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/ExecutionEngine/JITLink/ELF_x86_64.h"
+#include "llvm/ExecutionEngine/JITLink/aarch64.h"
 #include "llvm/ExecutionEngine/JITLink/x86_64.h"
 #include "llvm/ExecutionEngine/Orc/DebugUtils.h"
 #include "llvm/ExecutionEngine/Orc/ExecutionUtils.h"
 #include "llvm/Support/BinaryByteStream.h"
 #include "llvm/Support/Debug.h"
 
 #define DEBUG_TYPE "orc"
 
const auto &TT =
         ENP.getExecutionSession().getExecutorProcessControl().getTargetTriple();
 
     switch (TT.getArch()) {
     case Triple::x86_64:
       PointerSize = 8;
       Endianness = support::endianness::little;
       EdgeKind = jitlink::x86_64::Pointer64;
       break;
+    case Triple::aarch64:
+      PointerSize = 8;
+      Endianness = support::endianness::little;
+      EdgeKind = jitlink::aarch64::Pointer64;
+      break;
     default:
       llvm_unreachable("Unrecognized architecture");
     }
 
     // void *__dso_handle = &__dso_handle;
     auto G = std::make_unique<jitlink::LinkGraph>(
         "<DSOHandleMU>", TT, PointerSize, Endianness,
         jitlink::getGenericEdgeKindName);
if (Name.equals(SecName))
       return true;
   }
   return false;
 }
 
 bool ELFNixPlatform::supportedTarget(const Triple &TT) {
   switch (TT.getArch()) {
   case Triple::x86_64:
+  case Triple::aarch64:
     return true;
   default:
     return false;
   }
 }
 
 ELFNixPlatform::ELFNixPlatform(
     ExecutionSession &ES, ObjectLinkingLayer &ObjLinkingLayer,