SL-10291 Replace apr_atomic with standard C++11 functionality

indra/llcommon/CMakeLists.txt

indra/llcommon/llapp.h

indra/llcommon/llapr.h

indra/llcommon/llatomic.cpp

indra/llcommon/llatomic.h

indra/llcommon/llinstancetracker.cpp

indra/llcommon/llinstancetracker.h

indra/llcommon/llmutex.cpp

indra/llcommon/llqueuedthread.h

indra/llcommon/llrefcount.h

indra/llcommon/llworkerthread.h

indra/llcorehttp/_httpservice.h

indra/llcorehttp/_refcounted.h

indra/llcorehttp/_thread.h

indra/llcrashlogger/llcrashlock.cpp

indra/llmessage/llproxy.h

indra/llvfs/llpidlock.cpp

indra/newview/llappviewer.h

indra/newview/llmeshrepository.cpp

indra/newview/lltexturecache.h