[libc] Fix standalone cross compiling build for the GPU (#84042)

Summary:
This patch fixes some issues with building the GPU target manually
without the runtimes bootstrapping build. This fixes the install
directory and sets the default namespace to something more sensible if
not set. Also I got rid of the check on `-mcpu=native`. it was a neat
trick, but CMake in its INFINITE wisdom does not allow you to set link
flags on the compiler flag check. So I just went with the old tool
usage.

Author: jhuber6

libc/CMakeLists.txt

@@ -39,7 +39,11 @@ set(LIBC_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR})
 set(LIBC_ENABLE_USE_BY_CLANG OFF CACHE BOOL "Whether or not to place libc in a build directory findable by a just built clang")
 
 # Defining a global namespace to enclose all libc functions.
-set(LIBC_NAMESPACE "__llvm_libc_${LLVM_VERSION_MAJOR}_${LLVM_VERSION_MINOR}_${LLVM_VERSION_PATCH}_${LLVM_VERSION_SUFFIX}"
+set(default_namespace "__llvm_libc")
+if(LLVM_VERSION_MAJOR)
+  set(default_namespace "__llvm_libc_${LLVM_VERSION_MAJOR}_${LLVM_VERSION_MINOR}_${LLVM_VERSION_PATCH}_${LLVM_VERSION_SUFFIX}")
+endif()
+set(LIBC_NAMESPACE ${default_namespace}
   CACHE STRING "The namespace to use to enclose internal implementations. Must start with '__llvm_libc'."
 )
 
@@ -232,7 +236,13 @@ else()
     set(LIBC_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX})
   endif()
   if(LIBC_TARGET_OS_IS_GPU)
-    set(LIBC_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR}/${LLVM_DEFAULT_TARGET_TRIPLE})
+    if(LLVM_RUNTIMES_TARGET)
+      set(LIBC_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR}/${LLVM_RUNTIMES_TARGET})
+    elseif(LIBC_TARGET_TRIPLE)
+      set(LIBC_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR}/${LIBC_TARGET_TRIPLE})
+    else()
+      set(LIBC_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR}/${LLVM_DEFAULT_TARGET_TRIPLE})
+    endif()
   else()
     set(LIBC_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR})
   endif()

libc/cmake/modules/prepare_libc_gpu_build.cmake

@@ -48,10 +48,20 @@ endif()
 
 set(LIBC_GPU_TEST_ARCHITECTURE "" CACHE STRING "Architecture for the GPU tests")
 if(LIBC_TARGET_ARCHITECTURE_IS_AMDGPU)
-  check_cxx_compiler_flag("-nogpulib -mcpu=native" PLATFORM_HAS_GPU)
+  # Identify any locally installed NVIDIA GPUs on the system using 'nvptx-arch'.
+  find_program(LIBC_AMDGPU_ARCH
+               NAMES amdgpu-arch NO_DEFAULT_PATH
+               PATHS ${LLVM_BINARY_DIR}/bin ${compiler_path})
+  if(LIBC_AMDGPU_ARCH)
+    execute_process(COMMAND ${LIBC_AMDGPU_ARCH}
+                    OUTPUT_VARIABLE arch_tool_output
+                    ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)
+    if(arch_tool_output MATCHES "^gfx[0-9]+")
+      set(PLATFORM_HAS_GPU TRUE)
+    endif()
+  endif()
 elseif(LIBC_TARGET_ARCHITECTURE_IS_NVPTX)
   # Identify any locally installed NVIDIA GPUs on the system using 'nvptx-arch'.
-  # Using 'check_cxx_compiler_flag' does not work currently due to the link job.
   find_program(LIBC_NVPTX_ARCH
                NAMES nvptx-arch NO_DEFAULT_PATH
                PATHS ${LLVM_BINARY_DIR}/bin ${compiler_path})