This adds test coverage for builds that use the TSD dispatch stubs, but still use a __thread variable for the dispatch table. With !249 (merged), that's the combination you'd get on a non-glibc system.
To do that, I added a separate dispatch-tls option to the Meson build to disable the TLS dispatch stubs even if they'd otherwise be supported.
I also changed the tls option to be a boolean instead of a feature, since it shouldn't be affected by Meson's --auto-features.