The existing storage/rocksdb/CMakeCache.txt defined
ATOMIC_EXTRA_LIBS when atomics where required. This was
determined by the toplevel configure.cmake test
(HAVE_GCC_C11_ATOMICS_WITH_LIBATOMIC).

As build_rocksdb.cmake is included after ATOMIC_EXTRA_LIBS
was set, we just need to use it. As such no riscv64
specific macro is needed in build_rocksdb.cmake.

As highlighted by Gianfranco Costamagna (@LocutusOfBorg)
in #2472 overwriting SYSTEM_LIBS was problematic.
This is corrected in case in future SYSTEM_LIBS is changed
elsewhere.

Closes #2472.