Add cxxstdlib_check method to request changing C++ stdlib check.

* In certain cases, a C++ software may result in linking to a different
  or incompatible C++ standard library than its dependencies but still
  works fine or it is by design. Examples include GCC, which will
  bootstrap itself and self-host after finish.

* Add a cxxstdlib_check method to formula to request changing the C++
  standard library checking. Currently using "cxxstdlib_check :skip"
  will let a formula skip such checking. This should only be used on
  rare occasions and be very careful.

Closes #23687.