[libc] Fix GPU handling for unsupported backends (#92271)

Summary:
If the user does not have the selected backend enabled, we should still
be able to build the LLVM-IR an ddistribute it. This patch makes logic
to suppress tests if the backend can't build it, as well as removing a
flag for the building that's only present int he NVPTX backend.

Author: jhuber6

libc/cmake/modules/LLVMLibCCompileOptionRules.cmake

@@ -101,7 +101,6 @@ function(_get_common_compile_options output_var flags)
 
     if(LIBC_TARGET_ARCHITECTURE_IS_NVPTX)
       list(APPEND compile_options "-Wno-unknown-cuda-version")
-      list(APPEND compile_options "SHELL:-mllvm -nvptx-emit-init-fini-kernel=false")
       list(APPEND compile_options "--cuda-feature=+ptx63")
       if(LIBC_CUDA_ROOT)
         list(APPEND compile_options "--cuda-path=${LIBC_CUDA_ROOT}")

libc/cmake/modules/prepare_libc_gpu_build.cmake

@@ -76,7 +76,15 @@ elseif(LIBC_TARGET_ARCHITECTURE_IS_NVPTX)
 endif()
 
 set(gpu_test_architecture "")
-if(LIBC_GPU_TEST_ARCHITECTURE)
+if(DEFINED LLVM_TARGETS_TO_BUILD AND LIBC_TARGET_ARCHITECTURE_IS_AMDGPU
+   AND NOT "AMDGPU" IN_LIST LLVM_TARGETS_TO_BUILD)
+  set(LIBC_GPU_TESTS_DISABLED TRUE)
+  message(STATUS "AMDGPU backend is not available, tests will not be built")
+elseif(DEFINED LLVM_TARGETS_TO_BUILD AND LIBC_TARGET_ARCHITECTURE_IS_AMDGPU
+       AND NOT "NVPTX" IN_LIST LLVM_TARGETS_TO_BUILD)
+  set(LIBC_GPU_TESTS_DISABLED TRUE)
+  message(STATUS "NVPTX backend is not available, tests will not be built")
+elseif(LIBC_GPU_TEST_ARCHITECTURE)
   set(LIBC_GPU_TESTS_DISABLED FALSE)
   set(gpu_test_architecture ${LIBC_GPU_TEST_ARCHITECTURE})
   message(STATUS "Using user-specified GPU architecture for testing: "