GCC: No option to specify 'char' to be signed by default.

For ARM GCC 7.2.1, there is no option to specify 'char' to be signed by default. There is only an option to specify 'char' to be unsigned - which is pointless since 'char' is unsigned by default for ARM GCC. This existing GUI option should be replaced with an option to specify 'char' as signed (-fsigned-char)