RISCV64 port on Linux requires -latomic to be linked, as some
funcationality required by C/C++ may not be available in hardware,
requiring a runtime implementation.
Although, some compilers try to add -latomic, it may or may not work
under CMake and --as-needed linking.
This is similar to how some Android toolchain files specify -latomic
as well.
Tested on Ubuntu, with this patch fixes failure to build from source
of rocksdb, mariadb-10.3, opendht and probably other projects on
RISCV64 without modifying their CMake configuration.
