[libc++] Add GCC workaround in std::char_traits<char>::length()

GCC currently does not allow `__builtin_strlen()` during constant evaluation. This PR adds a workaround in `std::char_traits<char>::length()`

Reviewed By: Quuxplusone, ldionne, #libc, Mordante

Spies: Mordante, libcxx-commits

Differential Revision: https://reviews.llvm.org/D115795