[libc] Add `LIBC_GPU_TEST_ARCHITECTURE` option to set architecture

[jhuber6]

Summary

Currently, the plan is to support testing on a single GPU architecture.
We query the supported architectures from the user's system. However,
there are times when the user would want to override this. This patch
adds the LIBC_GPU_TEST_ARCHITECTURE option, which allows users to
specify which GPU architecture to build for.

libc/cmake/modules/prepare_libc_gpu_build.cmake

 find_program(LIBC_CLANG_OFFLOAD_PACKAGER
              NAMES clang-offload-packager
              PATHS ${LLVM_BINARY_DIR}/bin)
 if(NOT LIBC_CLANG_OFFLOAD_PACKAGER)
   message(FATAL_ERROR "Cannot find the 'clang-offload-packager' for the GPU "
                       "build")
 endif()
 
+set(LIBC_GPU_TEST_ARCHITECTURE "" CACHE STRING "Architecture for the GPU tests")
+if(LIBC_GPU_TEST_ARCHITECTURE)
+  message(STATUS "Using user-specified GPU architecture for testing "
+                 "'${LIBC_GPU_TARGET_ARCHITECTURE}'")
+  if("${LIBC_GPU_TARGET_ARCHITECTURE}" IN_LIST all_amdgpu_architectures)
+    set(LIBC_GPU_TARGET_ARCHITECTURE_IS_AMDGPU TRUE)
+    set(LIBC_GPU_TARGET_TRIPLE "amdgcn-amd-amdhsa")
+    set(LIBC_GPU_TARGET_ARCHITECTURE "${LIBC_GPU_TEST_ARCHITECTURE}")
+  elseif("${LIBC_GPU_TARGET_ARCHITECTURE}" IN_LIST all_nvptx_architectures)
+    set(LIBC_GPU_TARGET_ARCHITECTURE_IS_NVPTX TRUE)
+    set(LIBC_GPU_TARGET_TRIPLE "nvptx64-nvidia-cuda")
+    set(LIBC_GPU_TARGET_ARCHITECTURE "${LIBC_GPU_TEST_ARCHITECTURE}")
+  else()
+    message(FATAL_ERROR 
+            "Unknown GPU architecture '${LIBC_GPU_TARGET_ARCHITECTURE}'")
+  endif()
+  return()
+endif()
+
 # Identify any locally installed AMD GPUs on the system 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)
   execute_process(COMMAND ${LIBC_AMDGPU_ARCH}
                   OUTPUT_VARIABLE LIBC_AMDGPU_ARCH_OUTPUT
                   OUTPUT_STRIP_TRAILING_WHITESPACE)