[flang][RISCV][WIP] Add target-abi ModuleFlag.

This is needed to generate proper ABI flags in the ELF header for LTO builds.
If these flags aren't set correctly, we can't link with objects that
were built with the correct flags.

For non-LTO builds the mcpu/mattr in the TargetMachine will cause
the backend to infer an ABI. For LTO builds the mcpu/mattr aren't set.

Still need testing and validation of the ABI name.

Author: topperc

clang/lib/Driver/ToolChains/Flang.cpp

@@ -261,6 +261,10 @@ void Flang::AddPPCTargetArgs(const ArgList &Args,
 void Flang::AddRISCVTargetArgs(const ArgList &Args,
                                ArgStringList &CmdArgs) const {
   const llvm::Triple &Triple = getToolChain().getTriple();
+
+  StringRef ABIName = riscv::getRISCVABI(Args, Triple);
+  CmdArgs.push_back(Args.MakeArgString("-mabi=" + ABIName));
+
   // Handle -mrvv-vector-bits=<bits>
   if (Arg *A = Args.getLastArg(options::OPT_mrvv_vector_bits_EQ)) {
     StringRef Val = A->getValue();

flang/include/flang/Frontend/TargetOptions.h

@@ -35,6 +35,9 @@ class TargetOptions {
   /// If given, the name of the target CPU to tune code for.
   std::string cpuToTuneFor;
 
+  /// If given, the name of the target ABI to use.
+  std::string abi;
+
   /// The list of target specific features to enable or disable, as written on
   /// the command line.
   std::vector<std::string> featuresAsWritten;

flang/lib/Frontend/CompilerInvocation.cpp

@@ -464,6 +464,7 @@ static void parseTargetArgs(TargetOptions &opts, llvm::opt::ArgList &args) {
 
   if (const llvm::opt::Arg *a =
           args.getLastArg(clang::driver::options::OPT_mabi_EQ)) {
+    opts.abi = a->getValue();
     llvm::StringRef V = a->getValue();
     if (V == "vec-extabi") {
       opts.EnableAIXExtendedAltivecABI = true;

flang/lib/Frontend/FrontendActions.cpp

@@ -920,16 +920,23 @@ void CodeGenAction::generateLLVMIR() {
           static_cast<llvm::PIELevel::Level>(opts.PICLevel));
   }
 
+  const TargetOptions &targetOpts = ci.getInvocation().getTargetOpts();
+  const llvm::Triple triple(targetOpts.triple);
+
   // Set mcmodel level LLVM module flags
   std::optional<llvm::CodeModel::Model> cm = getCodeModel(opts.CodeModel);
   if (cm.has_value()) {
-    const llvm::Triple triple(ci.getInvocation().getTargetOpts().triple);
     llvmModule->setCodeModel(*cm);
     if ((cm == llvm::CodeModel::Medium || cm == llvm::CodeModel::Large) &&
         triple.getArch() == llvm::Triple::x86_64) {
       llvmModule->setLargeDataThreshold(opts.LargeDataThreshold);
     }
   }
+
+  if (triple.isRISCV() && !targetOpts.abi.empty())
+    llvmModule->addModuleFlag(
+        llvm::Module::Error, "target-abi",
+        llvm::MDString::get(llvmModule->getContext(), targetOpts.abi));
 }
 
 static std::unique_ptr<llvm::raw_pwrite_stream>