[flang][runtime] Let FORT_CHECK_POINTER_DEALLOCATION=0 disable runtime … (#84956)

…check

Add an environment variable by which a user can disable the pointer
validation check in DEALLOCATE statement handling. This is not safe, but
it can help make a code work that allocates a pointer with an extended
derived type, associates its target with a pointer to one of its
ancestor types, and then deallocates that pointer.