MDEV-30554 RockDB libatomic linking on riscv64

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.

storage/rocksdb/build_rocksdb.cmake

@@ -129,10 +129,6 @@ if(CMAKE_SYSTEM_PROCESSOR MATCHES "ppc64")
   ADD_DEFINITIONS(-DHAVE_POWER8 -DHAS_ALTIVEC)
 endif(CMAKE_SYSTEM_PROCESSOR MATCHES "ppc64")
 
-if(CMAKE_SYSTEM_PROCESSOR STREQUAL "riscv64")
- set(SYSTEM_LIBS ${SYSTEM_LIBS} -latomic)
-endif()
-
 option(WITH_FALLOCATE "build with fallocate" ON)
 
 if(WITH_FALLOCATE AND UNIX)
@@ -161,9 +157,9 @@ include_directories(SYSTEM ${ROCKSDB_SOURCE_DIR}/third-party/gtest-1.7.0/fused-s
 
 find_package(Threads REQUIRED)
 if(WIN32)
-  set(SYSTEM_LIBS ${SYSTEM_LIBS} Shlwapi.lib Rpcrt4.lib)
+  set(SYSTEM_LIBS ${SYSTEM_LIBS} ${ATOMIC_EXTRA_LIBS} Shlwapi.lib Rpcrt4.lib)
 else()
-  set(SYSTEM_LIBS ${CMAKE_THREAD_LIBS_INIT} ${LIBRT} ${CMAKE_DL_LIBS})
+  set(SYSTEM_LIBS ${SYSTEM_LIBS} ${CMAKE_THREAD_LIBS_INIT} ${LIBRT} ${CMAKE_DL_LIBS} ${ATOMIC_EXTRA_LIBS})
 endif()
 
 set(ROCKSDB_LIBS rocksdblib})