[libc] Add check for locally installed GPUs

[jhuber6]

Summary

We need to know which, if any, GPUs the user has on their system if we
want to be able to test the libc source code for the GPU. This patch
adds a basic check using the amdgpu-arch utility which is provided by
clang.

Checking for NVIDIA GPUs will be done later as this is a little
problematic right now. CMake provides a method that we use for Clang but
it will soon be deprecated, the replacement requires a newer CMake
version that we will have in the LLVM 17 branch in the future. CUDA also
provides __nvcc_device_query but it's very new so I'm not sure if we
should rely on it. I may introduce a new tool to do it similar to
amdgpu-arch.

libc/cmake/modules/prepare_libc_gpu_build.cmake

message(FATAL_ERROR "Cannot build libc for GPU. CMake compiler "
                       "'${CMAKE_CXX_COMPILER_ID} ${CMAKE_CXX_COMPILER_VERSION}' "
                       " is not `Clang ${req_ver}.")
 endif()
 if(NOT LLVM_LIBC_FULL_BUILD)
   message(FATAL_ERROR "LLVM_LIBC_FULL_BUILD must be enabled to build libc for "
                       "GPU.")
 endif()
 
+# Identify any locally installed GPUs to use for testing.
 find_program(LIBC_AMDGPU_ARCH
              NAMES amdgpu-arch
              PATHS ${LLVM_BINARY_DIR}/bin /opt/rocm/llvm/bin/)
-if (LIBC_AMDGPU_ARCH)
+if(LIBC_AMDGPU_ARCH)
   execute_process(COMMAND ${LIBC_AMDGPU_ARCH}
                   OUTPUT_VARIABLE LIBC_AMDGPU_ARCH_OUTPUT
                   OUTPUT_STRIP_TRAILING_WHITESPACE)
-  string(REPLACE "\n" ";" LIBC_AMDGPU_ARCH_OUTPUT "${LIBC_AMDGPU_ARCH_OUTPUT}")
-
-  list(LENGTH "${LIBC_AMDGPU_ARCH_OUTPUT}" LEN)
-  message(WARNING ${LEN})
-  message(WARNING ${LIBC_AMDGPU_ARCH_OUTPUT})
-  list(POP_BACK ${LIBC_AMDGPU_ARCH_OUTPUT} OUT)
-  message(WARNING ${OUT})
+  string(FIND "${LIBC_AMDGPU_ARCH_OUTPUT}" "\n" first_arch_string)
+  string(SUBSTRING "${LIBC_AMDGPU_ARCH_OUTPUT}" 0 ${first_arch_string}
+         arch_string)
+  if(arch_string)
+    set(LIBC_GPU_TARGET_TRIPLE "amdgcn-amd-amdhsa")
+    set(LIBC_GPU_TARGET_ARCHITECTURE "${arch_string}")
+  endif()
 endif()
+# TODO: Check for Nvidia GPUs.