[Driver] Force avr-ld to be default linker for target AVR

[haowei]

I tested this patch on top of https://github.com/llvm/llvm-project/commit/3b6e166999bb83197c8b70468c2d371c58cf0c25 (reverted in https://github.com/llvm/llvm-project/commit/7fae15f9251d3b392058e2fc84898b53619d36ad) and Clang driver tests now pass without issue.

[benshi001]

In D127814#3583854, @haowei wrote:

I tested this patch on top of https://github.com/llvm/llvm-project/commit/3b6e166999bb83197c8b70468c2d371c58cf0c25 (reverted in https://github.com/llvm/llvm-project/commit/7fae15f9251d3b392058e2fc84898b53619d36ad) and Clang driver tests now pass without issue.

Thanks. You are appreciated to also click accept in https://reviews.llvm.org/D126192 :)

clang/lib/Driver/ToolChains/AVR.cpp

void AVR::Linker::ConstructJob(Compilation &C, const JobAction &JA,
 
   // Compute information about the target AVR.
   std::string CPU = getCPUName(D, Args, getToolChain().getTriple());
   llvm::Optional<StringRef> FamilyName = GetMCUFamilyName(CPU);
   llvm::Optional<std::string> AVRLibcRoot = TC.findAVRLibcInstallation();
   llvm::Optional<unsigned> SectionAddressData = GetMCUSectionAddressData(CPU);
 
   // Compute the linker program path, and use GNU "avr-ld" as default.
-  std::string Linker = getToolChain().GetLinkerPath(nullptr);
+  const Arg *A = Args.getLastArg(options::OPT_fuse_ld_EQ);
+  std::string Linker = A ? getToolChain().GetLinkerPath(nullptr)
+                         : getToolChain().GetProgramPath(getShortName());
 
   ArgStringList CmdArgs;
   AddLinkerInputs(getToolChain(), Inputs, Args, CmdArgs, JA);
 
   CmdArgs.push_back("-o");
   CmdArgs.push_back(Output.getFilename());
 
   // Enable garbage collection of unused sections.