--code-coverage corrupts memory when run with multiple threads

[vtjnash]

Now that threads are the default, I've noticed that our coverage.cpp file is not thread-safe, since coverage wasn't supposed to be run using threads (it gets miscounted if run with threads, but also corrupts memory). However, the default options for threads got changed in v1.12, so now our coverage runs with threads enabled, which causes memory corruption on those CI jobs that use the default options.

[gbaraldi]

For 1.12 can we detect coverage + threads during initialization and fail gracefully? Or just disable threads directly?