Replace legacy sync* builtins with modern atomic* builtins: fixes build on 32-bit platforms

[barracuda156]

@rakshasa Could you please take a look?

The issue is that old sync* builtins require 8-byte atomics support in hardware. That leaves 32-bit platforms (arm, ppc, mips) broken. What will work for both 64- and 32-bit platforms alike is usage of modern atomic* builtins. 32-bit archs will just need an explicit linking to libatomic.

This a patch which is confirmed to fix compilation on ppc, but maybe you can suggest a better solution or verify/improve upon this one?

https://github.com/macports/macports-ports/blob/4ac3291874cad2051b9cc1ba08cadd62643f92f5/net/libtorrent-devel/files/0001-Fix-atomics-for-modern-gcc.patch

[stickz]

@barracuda156 Could you submit a pull request for this patch? I'd like to take responsibility for testing this change and getting it incorporated into the project. I'll request a review from @rakshasa when the time is right and get it merged for you.

[rakshasa]

I'd rather replace them with std::atomic, as that will avoid any compiler dependency.

[barracuda156]

@stickz Sorry, I am away from the testing hardware at the moment, so can’t do much until I am back.

@rakshasa If you can handle this, it will be much better, since I am not too sure that my fix is the right one.

(FWIW, we will still depend on libatomic on some 32-bit archs, AFAIU, as long as 8-byte atomics is needed in some form.)