Linking failure on RISC-V with GCC 11.2.1

libtorrent version (or branch): `2.0.5`



Platform/Architecture: RISC-V, Gentoo Linux, with `lp64d systemd` profile



Compiler and compiler version: `gcc (Gentoo 11.2.1_p20220115 p4) 11.2.1 20220115`



Encountered the problem while trying to compile the qBittorrent 4.4.2 ebuild with latest portage tree. `libtorrent-rasterbar` was built successfully, but during linking of qBittorrent it would fail due to missing `__atomic_*` symbol from `libtorrent`.



```



/usr/lib/gcc/riscv64-unknown-linux-gnu/11.2.1/../../../../riscv64-unknown-linux-gnu/bin/ld: /usr/lib64/libtorrent-rasterbar.

so.2.0.5: undefined reference to `__atomic_exchange_1'                                                                      

collect2: error: ld returned 1 exit status                                                                                  

distcc[2276441] ERROR: compile (null) on localhost failed

ninja: build stopped: subcommand failed. 



```



AFAIK, this is because CMake script fails to detect missing subword atomic, as RISC-V doesn't have 1/2 byte atomic instruction.



- GCC should provide subword atomic inlining IMO, but currently they don't.

- This was bypassed by prefer `pthread`, hints linking to `latomic` previously (#6722). However, after glibc 2.34, `-pthread` is nolonger needed to use `pthread` libraries, hence no more linking to `latomic`.



Since replying on `-pthread` for atomic is sementically incorrect, plus we already have a detection mechanism here, a possible fix could be enhance the detection.



Build log:

[libtorrent-rasterbar-2.0.5-r1.log](https://github.com/arvidn/libtorrent/files/8411284/libtorrent-rasterbar-2.0.5-r1_20220403-153624.log)

[qbittorrent-4.4.2.log](https://github.com/arvidn/libtorrent/files/8411287/qbittorrent-4.4.2_20220403-161029.log)

[]