[libc] Correctly find LLVM binaries when built in projects mode for GPU

Summary:
You can build the GPU libc support in projects mode. There were some
issues with it not finding the correct binaries. This patch fixes that.

libc/cmake/modules/prepare_libc_gpu_build.cmake

@@ -18,20 +18,28 @@ endif()
 
 # Identify the program used to package multiple images into a single binary.
 get_filename_component(compiler_path ${CMAKE_CXX_COMPILER} DIRECTORY)
-find_program(LIBC_CLANG_OFFLOAD_PACKAGER
-             NAMES clang-offload-packager NO_DEFAULT_PATH
-             PATHS ${LLVM_BINARY_DIR}/bin ${compiler_path})
-if(NOT LIBC_CLANG_OFFLOAD_PACKAGER)
-  message(FATAL_ERROR "Cannot find the 'clang-offload-packager' for the GPU "
-                      "build")
+if(TARGET clang-offload-packager)
+  get_target_property(LIBC_CLANG_OFFLOAD_PACKAGER clang-offload-packager LOCATION)
+else()
+  find_program(LIBC_CLANG_OFFLOAD_PACKAGER
+               NAMES clang-offload-packager NO_DEFAULT_PATH
+               PATHS ${LLVM_BINARY_DIR}/bin ${compiler_path})
+  if(NOT LIBC_CLANG_OFFLOAD_PACKAGER)
+    message(FATAL_ERROR "Cannot find the 'clang-offload-packager' for the GPU "
+                        "build")
+  endif()
 endif()
 
 # Identify llvm-link program so we can merge the output IR into a single blob.
-find_program(LIBC_LLVM_LINK
-             NAMES llvm-link NO_DEFAULT_PATH
-             PATHS ${LLVM_BINARY_DIR}/bin ${compiler_path})
-if(NOT LIBC_LLVM_LINK)
-  message(FATAL_ERROR "Cannot find 'llvm-link' for the GPU build")
+if(TARGET llvm-link)
+  get_target_property(LIBC_LLVM_LINK llvm-link LOCATION)
+else()
+  find_program(LIBC_LLVM_LINK
+               NAMES llvm-link NO_DEFAULT_PATH
+               PATHS ${LLVM_BINARY_DIR}/bin ${compiler_path})
+  if(NOT LIBC_LLVM_LINK)
+    message(FATAL_ERROR "Cannot find 'llvm-link' for the GPU build")
+  endif()
 endif()
 
 # Optionally set up a job pool to limit the number of GPU tests run in parallel.
@@ -48,10 +56,14 @@ endif()
 
 set(LIBC_GPU_TEST_ARCHITECTURE "" CACHE STRING "Architecture for the GPU tests")
 if(LIBC_TARGET_ARCHITECTURE_IS_AMDGPU)
-  # 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})
+  # Identify any locally installed AMD GPUs on the system using 'amdgpu-arch'.
+  if(TARGET amdgpu-arch)
+    get_target_property(LIBC_AMDGPU_ARCH amdgpu-arch LOCATION)
+  else()
+    find_program(LIBC_AMDGPU_ARCH
+                 NAMES amdgpu-arch NO_DEFAULT_PATH
+                 PATHS ${LLVM_BINARY_DIR}/bin ${compiler_path})
+  endif()
   if(LIBC_AMDGPU_ARCH)
     execute_process(COMMAND ${LIBC_AMDGPU_ARCH}
                     OUTPUT_VARIABLE arch_tool_output
@@ -62,9 +74,13 @@ if(LIBC_TARGET_ARCHITECTURE_IS_AMDGPU)
   endif()
 elseif(LIBC_TARGET_ARCHITECTURE_IS_NVPTX)
   # Identify any locally installed NVIDIA GPUs on the system using 'nvptx-arch'.
-  find_program(LIBC_NVPTX_ARCH
-               NAMES nvptx-arch NO_DEFAULT_PATH
-               PATHS ${LLVM_BINARY_DIR}/bin ${compiler_path})
+  if(TARGET nvptx-arch)
+    get_target_property(LIBC_NVPTX_ARCH nvptx-arch LOCATION)
+  else()
+    find_program(LIBC_NVPTX_ARCH
+                 NAMES nvptx-arch NO_DEFAULT_PATH
+                 PATHS ${LLVM_BINARY_DIR}/bin ${compiler_path})
+  endif()
   if(LIBC_NVPTX_ARCH)
     execute_process(COMMAND ${LIBC_NVPTX_ARCH}
                     OUTPUT_VARIABLE arch_tool_output