libc++ expects to be able to call _VSTD::signbit(__u) as part of <compare>.

https://github.com/llvm/llvm-project/blob/c12386ae247c0d46e1d513942e322e3a0510b126/libcxx/include/__compare/strong_order.h#L68

However, in our LibM/math.h, we #define signbit(x) __builtin_signbit(x).

This causes the code to look like return ::std::__builtin_signbit(__u), which is clearly nonsense.

Some things to check:

• Are we allowed to #define signbit per POSIX? Per the C standard? Per the C++ standard?
• Do we need to add some magic to our llvm patches to let libc++ know that we're going to #define all these things and it should undef them after including our math.h?

The impact of this is that a c++ file that just contains:

#include <utility> // or <compare>
int main() {}

compiled with -std=c++20 does not compile for serenity with Clang.

cc @BertalanD