GCC v4.4 does not implement __builtin_unreachable(), so avoid using it when building with this version or earlier. Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Reviewed-by: Jordan Justen <jordan.l.justen@intel.com> Tested-by: Laszlo Ersek <lersek@redhat.com> Tested-By: Liming Gao <liming.gao@intel.com> Reviewed-by: Liming Gao <liming.gao@intel.com>