ld.lld 16.0.0 and newer defaults to '--no-undefined-version', which
causes the following error when CONFIG_MIPS_CLOCK_VSYSCALL is not set:

  ld.lld: error: version script assignment of 'LINUX_2.6' to symbol '__vdso_gettimeofday' failed: symbol not defined

Only export __vdso_gettimeofday() when it will be present in the final
object file, which clears up the error.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308170532.zxFFv25c-lkp@intel.com/Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Fangrui Song <maskray@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>