Upgrade devel/libatomic_ops from 7.6.4 to 7.6.8

Changelog:

• Eliminate 'casting signed to bigger unsigned int' CSA warning (test_stack)
• Eliminate 'redundant blank line at start/end of block' CodeFactor warning
• Fix 'Cannot implement CAS_full on this architecture' build error (nios2)
• Fix a typo in arm_v6.h
• Support aarch64-ilp32 (GCC) and UWP/arm64 (MS VC) targets
• Undefine AO_ARM_HAVE_* private macros after their usage
• Use standalone private macro to guard against AO_GCC_BARRIER redefinition
• Workaround 'condition my_chunk_ptr is always false' cppcheck false positive