[AIX][LTO] Properly respect LDR_CNTRL and set MAXDATA32 to 0xA0000000@DSA

[w2yehia]

Summary

0xA0000000@DSA is the safest maximum we can set MAXDATA32 to before calling the system assembler.
Also fix the logic for appending any user defined value of the LDR_CNTRL variable (in the current logic the shell variable expansion does not occur, so the value of LDR_CNTRL ends up being MAXDATA32=0x80000000@${LDR_CNTRL})

[rzurob]

LGTM.

llvm/lib/LTO/LTOCodeGenerator.cpp

if (!llvm::AIXSystemAssemblerPath.empty()) {
     if (llvm::sys::fs::real_path(llvm::AIXSystemAssemblerPath, AssemblerPath,
                                  /* expand_tilde */ true)) {
       emitError(
           "Cannot find the assembler specified by lto-aix-system-assembler");
       return false;
     }
   }
 
+  // Setup the LDR_CNTRL variable
+  std::string LDR_CNTRL_var = "LDR_CNTRL=MAXDATA32=0xA0000000@DSA";
+  if (Optional<std::string> V = sys::Process::GetEnv("LDR_CNTRL"))
+    LDR_CNTRL_var += ("@" + *V);
+
   // Prepare inputs for the assember.
   const auto &Triple = TargetMach->getTargetTriple();
   const char *Arch = Triple.isArch64Bit() ? "-a64" : "-a32";
   std::string ObjectFileName(AssemblyFile);
   ObjectFileName[ObjectFileName.size() - 1] = 'o';
   SmallVector<StringRef, 8> Args = {
-      "/bin/env",     "LDR_CNTRL=MAXDATA32=0x80000000@${LDR_CNTRL}",
+      "/bin/env",     LDR_CNTRL_var,
       AssemblerPath,  Arch,
       "-many",        "-o",
       ObjectFileName, AssemblyFile};
 
   // Invoke the assembler.
   int RC = sys::ExecuteAndWait(Args[0], Args);
 
   // Handle errors.