[libc] Fix GPU fatbinary dependencies for multi-source object libraries

Summary:
Multi-source object libraries require some additional handling, this
logic wasn't correctly settending the dependency on each filename
individually and was instead using the last one. This meant that only
the last file was built for multi-object libraries.

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -139,7 +139,8 @@ function(_build_gpu_objects fq_target_name internal_target_name)
                                ${packager_images} -o ${packaged_output_name}
                        DEPENDS ${gpu_target_names} ${add_gpu_obj_src} ${ADD_GPU_OBJ_HDRS}
                        COMMENT "Packaging LLVM offloading binary")
-    add_custom_target(${packaged_target_name} DEPENDS ${packaged_output_name} ${gpu_target_name})
+    add_custom_target(${packaged_target_name} DEPENDS ${packaged_output_name})
+    list(APPEND packaged_gpu_names ${packaged_target_name})
     list(APPEND packaged_gpu_binaries ${packaged_output_name})
   endforeach()
 
@@ -177,7 +178,7 @@ function(_build_gpu_objects fq_target_name internal_target_name)
   endforeach()
   target_include_directories(${fq_target_name} PRIVATE ${include_dirs})
   add_dependencies(${fq_target_name}
-                   ${full_deps_list} ${packaged_target_name} ${stub_target_name})
+                   ${full_deps_list} ${packaged_gpu_names} ${stub_target_name})
 
   # We only build the internal target for a single supported architecture.
   if(LIBC_GPU_TARGET_ARCHITECTURE_IS_AMDGPU OR