Use C11 atomics instead of hand-rolled assembly

Another idea from @algr :

Use C11 atomics to implement our mutexes and other atomic operations, instead of using error-prone (#3956, e.g.) hand-rolled assembly.

I think it's just Windows that blocks this. We already require C++11 these days, but I didn't think Visual Studio supported many C11 features, including atomics. I just double-checked VS2017 and indeed there is no stdatomic.h. I don't have a VS2019 installation handy right now but searching online it does not look promising. Clang on Windows is a potential avenue but I believe there would be a lot of work to get there (there are many VS-specific compiler and linker flags and pragmas and other things in the DR code base that probably would need changing). Long-term that may be a good goal which would remove many of these missing language feature problems.