MdePkg: avoid __builtin_unreachable() on GCC v4.4

MdePkg: avoid __builtin_unreachable() on GCC v4.4

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>