[libc] Prevent system headers from being included for the GPU build

[jhuber6]

Summary

It's very important that the GPU build does not include any system
directories. We currently use -ffreestanding to disable a lot of these
features, but we can still accidentally include them if they are not
provided by libc yet. This patch changes this to use -nostdinc to
disable all standard search paths. Then we use the compiler's resource
directory to pick up the provided headers like stdint.h.

[JonChesterfield]

Fixes the build for me (previously, was trying to include inttypes from /usr)

libc/CMakeLists.txt

 # Retrieve the host compiler's resource dir.
 if(COMMAND_RETURN_CODE EQUAL 0)
   set(COMPILER_RESOURCE_DIR
     "${COMPILER_RESOURCE_DIR}" CACHE PATH "path to compiler resource dir"
   )
   message(STATUS "Set COMPILER_RESOURCE_DIR to "
                  "${COMPILER_RESOURCE_DIR} using --print-resource-dir")
 else()
+  if (LIBC_TARGET_ARCHITECTURE_IS_GPU)
+    message(FATAL_ERROR "COMPILER_RESOURCE_DIR must be set for GPU builds")
+  else()
     set(COMPILER_RESOURCE_DIR OFF)
     message(STATUS "COMPILER_RESOURCE_DIR not set
                     --print-resource-dir not supported by host compiler")
   endif()
+endif()
 
 option(LLVM_LIBC_FULL_BUILD "Build and test LLVM libc as if it is the full libc" OFF)
 option(LLVM_LIBC_IMPLEMENTATION_DEFINED_TEST_BEHAVIOR "Build LLVM libc tests assuming our implementation-defined behavior" ON)
 option(LLVM_LIBC_ENABLE_LINTING "Enables linting of libc source files" OFF)
 
 option(LIBC_GPU_BUILD "Build libc for the GPU. All CPU build options will be ignored." OFF)
 set(LIBC_TARGET_TRIPLE "" CACHE STRING "The target triple for the libc build.")
 

libc/cmake/modules/LLVMLibCObjectRules.cmake

elseif(MSVC)
     list(APPEND compile_options "/GR-")
     if(ADD_FMA_FLAG)
       list(APPEND compile_options "/arch:AVX2")
     endif()
   endif()
   if (LIBC_TARGET_ARCHITECTURE_IS_GPU)
     list(APPEND compile_options "-nogpulib")
     list(APPEND compile_options "-fvisibility=hidden")
+
+    # Manually disable all standard include paths and include the resource
+    # directory to prevent system headers from being included.
+    list(APPEND compile_options "-isystem${COMPILER_RESOURCE_DIR}/include")
+    list(APPEND compile_options "-nostdinc")
   endif()
   set(${output_var} ${compile_options} PARENT_SCOPE)
 endfunction()
 
 # Obtains NVPTX specific arguments for compilation.
 # The PTX feature is primarily based on the CUDA toolchain version. We want to
 # be able to target NVPTX without an existing CUDA installation, so we need to
 # set this manually. This simply sets the PTX feature to the minimum required