BaseTools/tools_def: enable Os optimization for GCC X64 builds

Now that we switched to the __builtin_ms_va_list VA_LIST type for
GCC/X64, we can trust the compiler to do the right thing even under
optimization, and so we can enable -Os optimization all the way back
to GCC44, and drop the -D define that prevents the use of the __builtin
VA_LIST types. Note that this requires the -maccumulate-outgoing-args
switch as well.

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>