[Bug libstdc++/67843] experimental/filesystem/iterators/directory_iterator.cc fails on armv5t

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67843

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 36458
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36458&action=edit
Require consistent shared_ptr lock policy

Maybe we should solve this by forcing a linker error when an object doesn't
have the same lock policy as the library.

This patch causes the shared_ptr tests to FAIL when run with -march=i386, with
this error:

include/bits/shared_ptr_base.h:160: undefined reference to `void
std::__check_lock_policy<(__gnu_cxx::_Lock_policy)1>()

Instead of corrupting the heap at run-time the tests fail to link because the
library only has a definition for 

std::__check_lock_policy<(__gnu_cxx::_Lock_policy)2>()

The same thing should work for arm and other targets where the availability of
atomics depends on -march.