Force default char to be signed on all architectures

We assume that `char` is `signed` in various places in the code. In particular, the `Str.StrToInts` test will fail when `char` is not `signed` and names containing special characters will be displayed incorrectly on servers.

Therefore, the compiler flag `-fsigned-char` is set unconditionally instead of only for ARM and ARM64, as we expect `char` to be `signed` on all architectures.

A static assertion is added to ensure at compile time that `char` is `signed` independently from the flag added in `CMakeLists.txt`.

This is necessary at least for ARM, ARM64, PPC, PPC64, and PPC64LE. According to some sources, `char` may also be `unsigned` by default when compiling for Android, although this could not be confirmed with the current Android NDK using Clang.

For the PowerPC architectures, Compiler Explorer can be used to confirm that `char` is not `signed` by default by checking whether the static assertion compiles (see https://godbolt.org/z/9rn5Mrf59) and that the assembly is different with the `-fsigned-char` flag (see https://godbolt.org/z/138zTj3Wa).

Closes #8386.

CMakeLists.txt

@@ -291,11 +291,10 @@ if(NOT MSVC AND NOT HAIKU)
     add_cxx_compiler_flag_if_supported(OUR_FLAGS -ffloat-store)
   endif()
 
-  # This is needed to get the server to correctly display special characters
-  # on ARM systems.
-  if("${CMAKE_SYSTEM_PROCESSOR}" MATCHES "arm" OR "${CMAKE_SYSTEM_PROCESSOR}" MATCHES "aarch64")
-    add_cxx_compiler_flag_if_supported(OUR_FLAGS -fsigned-char)
-  endif()
+  # We assume that char is signed in various places in the code. In particular,
+  # the Str.StrToInts test will fail when char is not signed and names containing
+  # special characters will be displayed incorrectly on servers.
+  add_cxx_compiler_flag_if_supported(OUR_FLAGS -fsigned-char)
 
   # Don't insert timestamps into PEs to keep the build reproducible.
   if(TARGET_OS STREQUAL "windows")

src/game/gamecore.cpp

@@ -9,6 +9,8 @@
 #include <base/system.h>
 #include <engine/shared/config.h>
 
+#include <limits>
+
 const char *CTuningParams::ms_apNames[] =
 	{
 #define MACRO_TUNING_PARAM(Name, ScriptName, Value, Description) #ScriptName,
@@ -63,6 +65,8 @@ float CTuningParams::GetWeaponFireDelay(int Weapon) const
 	}
 }
 
+static_assert(std::numeric_limits<char>::is_signed, "char must be signed for StrToInts to work correctly");
+
 void StrToInts(int *pInts, size_t NumInts, const char *pStr)
 {
 	dbg_assert(NumInts > 0, "StrToInts: NumInts invalid");