Add no_cxxstdlib_check method to request skip 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 no_cxxstdlib_check method to formula to request avoid the C++
  standard library check. This should only be used on rare occasions.

Closes #23687.
Use "cxxstdlib_check :skip" syntax.
Make cxxstdlib a set.