devel/binutils: 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 devel/binutils 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,
          |                                          ^
    ./stringpool.h:225:35: note: in instantiation of template class 'std::basic_string<unsigned short>' requested here
      225 |   { return this->add_with_length(s.data(), s.size(), copy, pkey); }
          |                                   ^
    stringpool.cc:530:7: note: in instantiation of member function 'gold::Stringpool_template<unsigned short>::add' requested here
      530 | class Stringpool_template<uint16_t>;
          |       ^
    /usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
       23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
          |                             ^

Upstream binutils has fixed this in commit 5e9091dab88 [2], so import it
as a separate patch, until the next version of binutils is released.

[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals
[2] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=5e9091dab88

PR:		280563
Approved by:	cy (maintainer)
MFH:		2024Q3

(cherry picked from commit 48aefee8)