[libc] Fix dependencies for generating the GPU binary file

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

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D144280

Author: jhuber6

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -132,30 +132,38 @@ function(_build_gpu_objects fq_target_name internal_target_name)
   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})
@@ -561,7 +569,6 @@ function(create_entrypoint_object fq_target_name)
     )
   endif()
 
-
   if(LLVM_LIBC_ENABLE_LINTING AND TARGET ${internal_target_name})
     if(NOT LLVM_LIBC_CLANG_TIDY)
       message(FATAL_ERROR "Something is wrong!  LLVM_LIBC_ENABLE_LINTING is "