Revert "MdePkg: avoid __builtin_unreachable() on GCC v4.4"

Revert "MdePkg: avoid __builtin_unreachable() on GCC v4.4"

This reverts commit 357cec385d4f ("MdePkg: avoid __builtin_unreachable()
on GCC v4.4", 2016-07-21).

We've removed BaseTools support for GCC44..GCC47, therefore we need not
catch the GCC44 corner case for __builtin_unreachable().

No GCC44..GCC47 references remain under MdePkg after this patch.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Marvin Haeuser <Marvin.Haeuser@outlook.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1377
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Marvin Haeuser <Marvin.Haeuser@outlook.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>