BaseTools: use unsigned chars on ARM architectures

BaseTools: use unsigned chars on ARM architectures

By default, the ARM architectures have unsigned chars, whereas the other
architectures supported by EDK2 by default have signed chars.
However, EDK2 uses -funsigned-chars on those architectures to change the
default behaviour.

Unfortunately, the ARM architectures explicitly break their default
behaviour by specifying -fsigned-chars (I presume in a pre-emptive
attempt at avoiding incompatibility).

Since this situation is already confusing enough, switch the ARM
architectures to also specify -funsigned-chars explicitly rather than
just dropping the current parameter.

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