Supress unused paramater warning conflicting with if constexpr

[basicer]

The version of g++ that github actions currently uses seems to see a parameter as unused if the only use is in if constexpr branch that are unused for a particular invocation.

./core/os/memory.h:200:45: error: parameter 'p_start' set but not used [-Werror=unused-but-set-parameter]
  200 | _FORCE_INLINE_ void memnew_arr_placement(T *p_start, size_t p_num) {
      |                                          ~~~^~~~~~~