[Bug libstdc++/63829] _Lock_policy used in thread.cc can cause incompatibilities with binaries using different -mcpu

[rearnsha at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63829

--- Comment #6 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
arm linux code should always be using the _S_atomic sequences.  When the
processor doesn't have the required instructions, kernel helper routines will
be used.  This ensures that code is forwards compatible to later architectures.
 It's a really bad idea for the headers used in user code to be changing this
decision dynamically.