[libc] Add an override option for specifying the loader implementation

[jhuber6]

Summary

There are some cases when testing we want to override the logic for not
building tests if the loader is not present. This allows users to
specify an external binary that fulfils the same duties which will force
the tests to be built even without meeting the dependencies.

[JonChesterfield]

Works a charm. Tested as "-DLIBC_GPU_LOADER_EXECUTABLE=$HOME/GPU/amdhsa_loader", switches on the tests as hoped. Thanks!

libc/utils/gpu/loader/CMakeLists.txt

message(WARNING
       "Skipping CUDA loader for gpu target, CUDA must be version 11.2 or later.
        Found CUDA Version ${CUDAToolkit_VERSION}")
   else()
     message(STATUS "Skipping CUDA loader for gpu target, no CUDA was detected")
   endif()
 endif()
 
 # Add a custom target to be used for testing.
-if(TARGET amdhsa_loader AND LIBC_GPU_TARGET_ARCHITECTURE_IS_AMDGPU)
+set(LIBC_GPU_LOADER_EXECUTABLE "" CACHE STRING "Overriding binary for the GPU loader.")
+if(LIBC_GPU_LOADER_EXECUTABLE)
+  add_custom_target(libc.utils.gpu.loader)
+  set_target_properties(
+    libc.utils.gpu.loader
+    PROPERTIES
+      EXECUTABLE "${LIBC_GPU_LOADER_EXECUTABLE}"
+  )
+elseif(TARGET amdhsa_loader AND LIBC_GPU_TARGET_ARCHITECTURE_IS_AMDGPU)
   add_custom_target(libc.utils.gpu.loader)
   add_dependencies(libc.utils.gpu.loader amdhsa_loader)
   set_target_properties(
     libc.utils.gpu.loader
     PROPERTIES
       EXECUTABLE "$<TARGET_FILE:amdhsa_loader>"
   )
 elseif(TARGET nvptx_loader AND LIBC_GPU_TARGET_ARCHITECTURE_IS_NVPTX)
   add_custom_target(libc.utils.gpu.loader)
   add_dependencies(libc.utils.gpu.loader nvptx_loader)
   set_target_properties(
     libc.utils.gpu.loader
     PROPERTIES
       EXECUTABLE "$<TARGET_FILE:nvptx_loader>"
   )
 endif()