[Xtensa] Default to unsigned char

This matches GCC.

Partially addresses llvm#115964

Pull Request: llvm#115967

Author: arichardson

clang/lib/Driver/ToolChains/Clang.cpp

@@ -1334,6 +1334,7 @@ static bool isSignedCharDefault(const llvm::Triple &Triple) {
   case llvm::Triple::riscv64:
   case llvm::Triple::systemz:
   case llvm::Triple::xcore:
+  case llvm::Triple::xtensa:
     return false;
   }
 }

clang/test/Driver/xtensa-char.c

@@ -0,0 +1,4 @@
+/// Check that char is unsigned by default.
+// RUN: %clang -### %s --target=xtensa -c 2>&1 | FileCheck %s
+// CHECK: "-cc1" "-triple" "xtensa"
+// CHECK-SAME: "-fno-signed-char"