Log Message:
libatomic_ops: update to 7.6.4.

== [7.6.4] 2018-03-27 ==

* Add RISC-V support
* Convert atomic_ops_malloc.c and tests to valid C++ code
* Eliminate 'function is never used' cppcheck warning for load_before_cas
* Eliminate 'using argument that points at uninitialized var' cppcheck error
* Fix 'AO_pt_lock undefined' error if cross-compiling manually (MinGW)
* Fix public headers inclusion from clients C++ code
* Remove gcc/nios2.h file (include gcc/generic.h directly for nios2)
* Support MIPS rel6