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

contrib/llvm-project/libcxx/src/expected.cpp

@@ -9,5 +9,7 @@
 #include <expected>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
+#if _LIBCPP_STD_VER >= 23
 const char* bad_expected_access<void>::what() const noexcept { return "bad access to std::expected"; }
+#endif
 _LIBCPP_END_NAMESPACE_STD

lib/libc++/Makefile

@@ -104,7 +104,15 @@ CFLAGS+=	-ffunction-sections
 CFLAGS+=	-fno-semantic-interposition
 CFLAGS+=	-fvisibility-inlines-hidden
 CFLAGS+=	-fvisibility=hidden
+
+.if ${COMPILER_TYPE} == "gcc" && ${COMPILER_VERSION} < 130000
+# NOTE: gcc 12 cannot correctly compile all libc++'s C++23 code. To temporarily
+# support gcc 12, compile libc++ in C++20 mode, but this will leave out any
+# C++23 features.
+CXXSTD?=	c++20
+.else
 CXXSTD?=	c++23
+.endif
 
 LIBADD+=	cxxrt