[Clang] Make the GPU toolchains implicitly link -lm and -lc

Summary:
The previous patches (The other commits in this chain) allow the
offloading toolchain to directly invoke the device linker. Because of
this, we can now just have the toolchain implicitly include `-lc` and
`-lm` like a standard target does. This removes the old handling that
went through the fat binary `-lcgpu`.

Author: jhuber6

clang/lib/Driver/ToolChains/AMDGPU.cpp

@@ -633,6 +633,17 @@ void amdgpu::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   else if (Args.hasArg(options::OPT_mcpu_EQ))
     CmdArgs.push_back(Args.MakeArgString(
         "-plugin-opt=mcpu=" + Args.getLastArgValue(options::OPT_mcpu_EQ)));
+
+  // If the user's toolchain has the 'include/amdgcn-amd-amdhsa/` path, we
+  // assume it supports the standard C libraries for the GPU and include them.
+  bool HasLibC = getToolChain().getStdlibIncludePath().has_value();
+  if (!Args.hasArg(options::OPT_nogpulib) &&
+      !Args.hasArg(options::OPT_nolibc) &&
+      Args.hasFlag(options::OPT_gpulibc, options::OPT_nogpulibc, HasLibC)) {
+    CmdArgs.push_back("-lc");
+    CmdArgs.push_back("-lm");
+  }
+
   CmdArgs.push_back("-o");
   CmdArgs.push_back(Output.getFilename());
   C.addCommand(std::make_unique<Command>(

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -1257,6 +1257,7 @@ bool tools::addOpenMPRuntime(const Compilation &C, ArgStringList &CmdArgs,
     addOpenMPDeviceLibC(C, Args, CmdArgs);
 
   addArchSpecificRPath(TC, Args, CmdArgs);
+
   addOpenMPRuntimeLibraryPath(TC, Args, CmdArgs);
 
   return true;

clang/lib/Driver/ToolChains/Cuda.cpp

@@ -617,6 +617,16 @@ void NVPTX::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     addLTOOptions(getToolChain(), Args, CmdArgs, Output, Inputs[0],
                   C.getDriver().getLTOMode() == LTOK_Thin);
 
+  // If the user's toolchain has the 'include/nvptx64-nvidia-cuda/` path, we
+  // assume it supports the standard C libraries for the GPU and include them.
+  bool HasLibC = getToolChain().getStdlibIncludePath().has_value();
+  if (!Args.hasArg(options::OPT_nogpulib) &&
+      !Args.hasArg(options::OPT_nolibc) &&
+      Args.hasFlag(options::OPT_gpulibc, options::OPT_nogpulibc, HasLibC)) {
+    CmdArgs.push_back("-lc");
+    CmdArgs.push_back("-lm");
+  }
+
   // Add paths for the default clang library path.
   SmallString<256> DefaultLibPath =
       llvm::sys::path::parent_path(TC.getDriver().Dir);