BaseTools AARCH64: move DEBUG GCC49 to the small code model

BaseTools AARCH64: move DEBUG GCC49 to the small code model

When building AARCH64 platforms that include a Shell binary built from
source, we run into trouble when using the tiny code model for DEBUG
builds. The reason is that the Shell binary built in DEBUG mode exceeds
the 1 MB range of the ADR instruction, so anything that gets pulled into
the final link of the Shell binary either needs to be built with the small
or large model, or needs to be sorted in some way to put the ADR references
close to their targets.

Since code size is not a big concern for DEBUG builds anyway, let's move
to the small code model for all modules when using DEBUG GCC49. This way,
there is no need for workarounds that are specific to UEFI_APPLICATION
modules in general, or the Shell application in particular.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>