[JITLink][ELF] Fix reading target architecture when the ELF object is big-endian

[lhames]

LGTM. Thanks @lkail!

llvm/lib/ExecutionEngine/JITLink/ELF.cpp

if (Data[ELF::EI_CLASS] == ELF::ELFCLASS64) {
       if (auto File = llvm::object::ELF32LEFile::create(Buffer)) {
         return File->getHeader().e_machine;
       } else {
         return File.takeError();
       }
     }
   }
 
+  if (Data[ELF::EI_DATA] == ELF::ELFDATA2MSB) {
+    if (Data[ELF::EI_CLASS] == ELF::ELFCLASS64) {
+      if (auto File = llvm::object::ELF64BEFile::create(Buffer)) {
+        return File->getHeader().e_machine;
+      } else {
+        return File.takeError();
+      }
+    } else if (Data[ELF::EI_CLASS] == ELF::ELFCLASS32) {
+      if (auto File = llvm::object::ELF32BEFile::create(Buffer)) {
+        return File->getHeader().e_machine;
+      } else {
+        return File.takeError();
+      }
+    }
+  }
+
   return ELF::EM_NONE;
 }
 
 Expected<std::unique_ptr<LinkGraph>>
 createLinkGraphFromELFObject(MemoryBufferRef ObjectBuffer) {
   StringRef Buffer = ObjectBuffer.getBuffer();
   if (Buffer.size() < ELF::EI_NIDENT)
     return make_error<JITLinkError>("Truncated ELF buffer");

llvm/test/ExecutionEngine/JITLink/ppc64/ppc64-relocs.s

 # RUN: llvm-mc -triple=powerpc64le-unknown-linux-gnu -filetype=obj -o %t %s
 # RUN: llvm-jitlink -abs external_var=0xffff0000 -abs puts=0xffff6400 -abs \
 # RUN:   foo=0xffff8800 -noexec %t
 # RUN: llvm-mc -triple=powerpc64-unknown-linux-gnu -filetype=obj -o %t %s
-# RUN: not llvm-jitlink -abs external_var=0xffff0000 -abs puts=0xffff6400 -abs \
+# RUN: llvm-jitlink -abs external_var=0xffff0000 -abs puts=0xffff6400 -abs \
 # RUN:   foo=0xffff8800 -noexec %t
 #
 # Check typical relocations involving external function call, external variable
 # reference, local function call and referencing global variable defined in the
 # same CU. This test serves as smoke test, `llvm-jitlink -check` is not used.
 
   .text
   .abiversion 2