Fix buildworld with gcc 12 after llvm-19 import
Unfortunately gcc 12's is not yet capable of compiling all of libc++
19's C++23 code, which results in errors similar to:
/usr/src/freebsd/src/contrib/llvm-project/libcxx/include/__algorithm/ranges_contains.h:41:3: error: 'static constexpr bool std::__1::ranges::__contains::__fn::operator()(_Iter, _Sent, const _Type&, _Proj)' must be a non-static member function
41 | operator()(_Iter __first, _Sent __last, const _Type& __value, _Proj __proj = {}) {
| ^~~~~~~~
/usr/src/freebsd/src/contrib/llvm-project/libcxx/include/__algorithm/ranges_contains.h:48:3: error: 'static constexpr bool std::__1::ranges::__contains::__fn::operator()(_Range&&, const _Type&, _Proj)' must be a non-static member function
48 | operator()(_Range&& __range, const _Type& __value, _Proj __proj = {}) {
| ^~~~~~~~Until we can get rid of gcc 12, work around this by making it compile
libc++ in C++20 mode instead.
NOTE: The resulting libc++ library will not be C++23 compatible! Please
try to avoid shipping it, and use gcc 13 instead, if you must use gcc.
PR: 280562
MFC after: 3 days
(cherry picked from commit bc9e19dce0abee80750e6fa04aaf979873bfe0d2)