[libc] Fix incorrect check for NVPTX backend

Author: jhuber6

libc/cmake/modules/prepare_libc_gpu_build.cmake

@@ -51,7 +51,7 @@ 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
+elseif(DEFINED LLVM_TARGETS_TO_BUILD AND LIBC_TARGET_ARCHITECTURE_IS_NVPTX
        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")