Standardize on -fsigned-char

MaGuess on Discord pointed out the fact that cosmocc contradicts itself
on the signedness of `char`. It's up to each platform to choose one, so
the cosmo platform shall choose signed. The rationale is it makes the C
language syntax more internally similar. `char` should be `signed char`
for the same reason `int` means `signed int`. It's recommended that you
still assume `char` could go either way since that's portable thinking.
But if you want to assume we'll always have signed char, that's ok too.

build/definitions.mk

@@ -127,6 +127,7 @@ ifeq ($(ARCH), aarch64)
 DEFAULT_COPTS +=							\
 	-ffixed-x18							\
 	-ffixed-x28							\
+	-fsigned-char							\
 	-mno-outline-atomics
 endif