[Clang] Add a flag to include GPU startup files

Summary:
The C library for GPUs provides the ability to target regular C/C++
programs by providing the C library and a file containing kernels that
call the `main` function. This is mostly used for unit tests, this patch
provides a quick way to add them without needing to know the paths. I
currently do this explicitly, but according to the libc++ contributors
we don't want to need to specify these paths manually. See the
discussion in #104515.

I just default to `lib/` if the target-specific one isn't found because
the linker will handle giving a reasonable error message if it's not
found. Basically the use-case looks like this.

```console
$ clang test.c --target=amdgcn-amd-amdhsa -mcpu=native -gpustartfiles
$ amdhsa-loader a.out
PASS!
```

Author: jhuber6

clang/include/clang/Driver/Options.td

@@ -1316,6 +1316,9 @@ defm offload_via_llvm : BoolFOption<"offload-via-llvm",
   BothFlags<[], [ClangOption], " LLVM/Offload as portable offloading runtime.">>;
 }
 
+def gpustartfiles : Flag<["-"], "gpustartfiles">, Group<Link_Group>,
+  HelpText<"Link the GPU C startup utilities automatically, used for testing.">;
+
 // CUDA options
 let Group = cuda_Group in {
 def cuda_include_ptx_EQ : Joined<["--"], "cuda-include-ptx=">,

clang/lib/Driver/ToolChains/AMDGPU.cpp

@@ -648,6 +648,15 @@ void amdgpu::Linker::ConstructJob(Compilation &C, const JobAction &JA,
         Args.MakeArgString("-plugin-opt=-mattr=" + llvm::join(Features, ",")));
   }
 
+  if (Args.hasArg(options::OPT_gpustartfiles)) {
+    auto IncludePath = getToolChain().getStdlibPath();
+    if (!IncludePath)
+      IncludePath = "/lib";
+    SmallString<128> P(*IncludePath);
+    llvm::sys::path::append(P, "crt1.o");
+    CmdArgs.append({"-lc", "-lm", Args.MakeArgString(P)});
+  }
+
   CmdArgs.push_back("-o");
   CmdArgs.push_back(Output.getFilename());
   C.addCommand(std::make_unique<Command>(

clang/lib/Driver/ToolChains/Cuda.cpp

@@ -641,6 +641,15 @@ void NVPTX::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   llvm::sys::path::append(DefaultLibPath, CLANG_INSTALL_LIBDIR_BASENAME);
   CmdArgs.push_back(Args.MakeArgString(Twine("-L") + DefaultLibPath));
 
+  if (Args.hasArg(options::OPT_gpustartfiles)) {
+    auto IncludePath = getToolChain().getStdlibPath();
+    if (!IncludePath)
+      IncludePath = "/lib";
+    SmallString<128> P(*IncludePath);
+    llvm::sys::path::append(P, "crt1.o");
+    CmdArgs.append({"-lc", "-lm", Args.MakeArgString(P)});
+  }
+
   C.addCommand(std::make_unique<Command>(
       JA, *this,
       ResponseFileSupport{ResponseFileSupport::RF_Full, llvm::sys::WEM_UTF8,

clang/test/Driver/gpustartfiles.c

@@ -0,0 +1,7 @@
+// RUN: %clang -target nvptx64-nvidia-cuda -march=sm_61 -gpustartfiles \
+// RUN:   -nogpulib -nogpuinc -### %s 2>&1 | FileCheck -check-prefix=NVPTX %s
+// NVPTX: clang-nvlink-wrapper{{.*}}"-lc" "-lm" "{{.*}}crt1.o"
+//
+// RUN: %clang -target amdgcn-amd-amdhsa -march=gfx90a -gpustartfiles \
+// RUN:   -nogpulib -nogpuinc -### %s 2>&1 | FileCheck -check-prefix=AMDGPU %s
+// AMDGPU: ld.lld{{.*}}"-lc" "-lm" "{{.*}}crt1.o"