[Xboxkrnl] Use towupper in RtlUpcaseUnicodeChar for portability

std::ctype<char16_t> is a non-standard extension that's not available
on all platforms. Neither this nor the original technically match the
Windows kernel's fixed Unicode table approach and can produce different
results based on system locale.

src/xenia/kernel/xboxkrnl/xboxkrnl_rtl.cc

@@ -229,8 +229,8 @@ void RtlInitAnsiString_entry(pointer_t<X_ANSI_STRING> destination,
 DECLARE_XBOXKRNL_EXPORT1(RtlInitAnsiString, kNone, kImplemented);
 // https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/wdm/nf-wdm-rtlupcaseunicodechar
 dword_result_t RtlUpcaseUnicodeChar_entry(dword_t SourceCharacter) {
-  return std::use_facet<std::ctype<char16_t>>(std::locale())
-      .toupper(SourceCharacter);
+  return static_cast<uint32_t>(std::towupper(
+      static_cast<wint_t>(static_cast<uint32_t>(SourceCharacter))));
 }
 DECLARE_XBOXKRNL_EXPORT1(RtlUpcaseUnicodeChar, kNone, kImplemented);