[CSKY] Default to unsigned char

This matches the ABI document found at
https://github.com/c-sky/csky-doc/blob/master/C-SKY_V2_CPU_Applications_Binary_Interface_Standards_Manual.pdf

Partially addresses https://github.com/llvm/llvm-project/issues/115957

Reviewed By: zixuan-wu

Pull Request: https://github.com/llvm/llvm-project/pull/115961

clang/lib/Driver/ToolChains/Clang.cpp

@@ -1387,6 +1387,7 @@ static bool isSignedCharDefault(const llvm::Triple &Triple) {
       return true;
     return false;
 
+  case llvm::Triple::csky:
   case llvm::Triple::hexagon:
   case llvm::Triple::msp430:
   case llvm::Triple::ppcle:

clang/test/Driver/csky-toolchain.c

@@ -3,6 +3,7 @@
 
 // RUN: %clang -### %s --target=csky 2>&1 | FileCheck -check-prefix=CC1 %s
 // CC1: "-cc1" "-triple" "csky"
+// CC1: "-fno-signed-char"
 
 // In the below tests, --rtlib=platform is used so that the driver ignores
 // the configure-time CLANG_DEFAULT_RTLIB option when choosing the runtime lib