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
 

tool/cosmocc/bin/cosmocc

@@ -265,7 +265,7 @@ LDFLAGS_X86_64="$LDFLAGS -L$BIN/../x86_64-linux-cosmo/lib -Wl,-T,$BIN/../x86_64-
 LDLIBS_X86_64="-lcosmo"
 
 CRT_AARCH64="$BIN/../aarch64-linux-cosmo/lib/crt.o"
-CPPFLAGS_AARCH64="$CPPFLAGS"
+CPPFLAGS_AARCH64="$CPPFLAGS -fsigned-char"
 CFLAGS_AARCH64="$CFLAGS -ffixed-x18 -ffixed-x28 -mno-outline-atomics"
 LDFLAGS_AARCH64="$LDFLAGS -L$BIN/../aarch64-linux-cosmo/lib -Wl,-T,$BIN/../aarch64-linux-cosmo/lib/aarch64.lds -Wl,-z,common-page-size=16384 -Wl,-z,max-page-size=16384"
 LDLIBS_AARCH64="-lcosmo"

tool/cosmocc/bin/cosmocross

@@ -84,6 +84,7 @@ if [ x"$ARCH" = x"x86_64" ]; then
 elif [ x"$ARCH" = x"aarch64" ]; then
   OBJCOPYFLAGS="-S"
   PAGESZ=16384
+  CPPFLAGS="$CPPFLAGS -fsigned-char"
   CFLAGS="$CFLAGS -ffixed-x18 -ffixed-x28 -mno-outline-atomics"
   LDFLAGS="$LDFLAGS -Wl,-T,$BIN/../$ARCH-linux-cosmo/lib/aarch64.lds"
 else