undefined reference to `__atomic_load_8' on ARMv6

Hello!

I encountered the following linker problem with the [recent](https://github.com/PurpleI2P/i2pd/releases/tag/2.14.0) i2pd sources:



```

HTTPServer.cpp:(.text+0x7e2c): undefined reference to `__atomic_load_8'

I2PControl.cpp:(.text+0xddc): undefined reference to `__atomic_load_8'

I2PControl.cpp:(.text+0xe74): undefined reference to `__atomic_load_8'

NTCPSession.cpp:(.text+0x5a00): undefined reference to `__atomic_fetch_add_8'

TransitTunnel.cpp:(.text+0x5c): undefined reference to `__atomic_fetch_add_8'

Transports.cpp:(.text+0x4bc): undefined reference to `__atomic_load_8'

SSUSession.cpp:(.text+0x370): undefined reference to `__atomic_fetch_add_8'

```

and so on.

So I had to add the `-latomic` to the following [line](https://github.com/PurpleI2P/i2pd/blob/openssl/build/CMakeLists.txt#L443) in the `CMakeLists.txt` in order to build the executable:



`target_link_libraries( "${PROJECT_NAME}" libi2pd i2pdclient ${DL_LIB} ${Boost_LIBRARIES} ${OPENSSL_LIBRARIES} ${ZLIB_LIBRARY} ${CMAKE_THREAD_LIBS_INIT} ${MINGW_EXTRA} ${DL_LIB} -latomic )`



I doubt this is a correct solution -- just reporting the result. This issue looks very similar to [another](https://github.com/PurpleI2P/i2pd/issues/285) one which was closed some time ago, but for another CPU architecture. So it might be worth making some kind of a universal solution to it.

[]