emulators/dosbox-x: Fix build with libc++ 19
As noted in the libc++ 19 release notes [1], std::char_traits<> is now
only provided for char, char8_t, char16_t, char32_t and wchar_t, and any
instantiation for other types will fail.
This causes emulators/dosbox-x to fail to compile with clang 19 and
libc++ 19, resulting in errors similar to:
/usr/include/c++/v1/string:820:42: error: implicit instantiation of undefined template 'std::char_traits<unsigned short>'
820 | static_assert(is_same<_CharT, typename traits_type::char_type>::value,
| ^
dos_programs.cpp:7692:17: note: in instantiation of template class 'std::basic_string<unsigned short>' requested here
7692 | test_string dst;
| ^
/usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
| ^
dos_programs.cpp:8893:14: warning: variable 'open' set but not used [-Wunused-but-set-variable]
8893 | bool open=false;
| ^This can be fixed by using char16_t for the 'test_char_t` type, and by
adding a few inline wrappers to perform the required casting.
[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals
PR: 282386
MFH: 2024Q4
(cherry picked from commit 573cdb2e7e34d6e97828e0b42456b17fc7c13f4b)