[libc] Fix dependencies for generating the GPU binary file

[jhuber6]

Summary

This patch adjusts the way dependencies are handled in the packaed
version of the GPU libc runtime. Previously the files were not getting
updated properly in the install when they changed.

Depends on D144214

[jhuber6]

Fix the filepath changing.

[jhuber6]

Fix deps on the stub making command. Works reliably now.

libc/cmake/modules/LLVMLibCObjectRules.cmake

function(_build_gpu_objects fq_target_name internal_target_name)
   # into a fatbinary, see https://clang.llvm.org/docs/OffloadingDesign.html for
   # more information.
   set(packaged_target_name ${fq_target_name}.__gpu__)
   set(packaged_output_name ${CMAKE_CURRENT_BINARY_DIR}/${fq_target_name}.gpubin)
 
   add_custom_command(OUTPUT ${packaged_output_name}
                      COMMAND ${LIBC_CLANG_OFFLOAD_PACKAGER}
                              ${packager_images} -o ${packaged_output_name}
-                     DEPENDS ${gpu_target_names}
+                     DEPENDS ${gpu_target_names} ${ADD_GPU_OBJ_SRCS} ${ADD_GPU_OBJ_HDRS}
                      COMMENT "Packaging LLVM offloading binary")
-  add_custom_target(${packaged_target_name} DEPENDS ${packaged_output_name})
+  add_custom_target(${packaged_target_name} DEPENDS ${packaged_output_name} ${gpu_target_name})
 
   # We create an empty 'stub' file for the host to contain the embedded device
   # code. This will be packaged into 'libcgpu.a'.
   # TODO: In the future we will want to combine every architecture for a target
   #       into a single bitcode file and use that. For now we simply build for
   #       every single one and let the offloading linker handle it.
   get_filename_component(stub_filename ${ADD_GPU_OBJ_SRCS} NAME)
-  file(MAKE_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/stubs)
-  file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/stubs/${stub_filename} "// Empty file.\n")
+  add_custom_command(
+    OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/stubs/${stub_filename}"
+    COMMAND ${CMAKE_COMMAND} -E make_directory ${CMAKE_CURRENT_BINARY_DIR}/stubs/
+    COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_CURRENT_BINARY_DIR}/stubs/${stub_filename}
+    DEPENDS ${gpu_target_names} ${ADD_GPU_OBJ_SRCS} ${ADD_GPU_OBJ_HDRS}
+  )
+  set(stub_target_name ${fq_target_name}.__stub__)
+  add_custom_target(${stub_target_name} DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/stubs/${stub_filename})
+
   add_library(
     ${fq_target_name}
     # We want an object library as the objects will eventually get packaged into
     # an archive (like libcgpu.a).
     EXCLUDE_FROM_ALL
     OBJECT
-    "${CMAKE_CURRENT_BINARY_DIR}/stubs/${stub_filename}"
+    ${CMAKE_CURRENT_BINARY_DIR}/stubs/${stub_filename}
   )
   target_compile_options(${fq_target_name} BEFORE PRIVATE ${common_compile_options}
                          -nostdlib -Xclang -fembed-offload-object=${packaged_output_name})
   target_include_directories(${fq_target_name} PRIVATE ${include_dirs})
-  add_dependencies(${fq_target_name} ${full_deps_list} ${packaged_target_name})
+  add_dependencies(${fq_target_name} 
+                   ${full_deps_list} ${packaged_target_name} ${stub_target_name})
 
   # We only build the internal target for a single supported architecture.
   set(include_dirs ${LIBC_BUILD_DIR}/include ${LIBC_SOURCE_DIR} ${LIBC_BUILD_DIR})
   if(LIBC_GPU_TARGET_ARCHITECTURE_IS_AMDGPU OR
      LIBC_GPU_TARGET_ARCHITECTURE_IS_NVPTX)
     add_library(
       ${internal_target_name}
       EXCLUDE_FROM_ALL