Previously there was only `--single-threaded`.
This flag now matches other boolean flags, instead of only being able to
opt in to single-threaded builds, you can now force multi-threaded
builds. Currently this only has the possibility to emit an error
message, but it is a better user experience to understand why one cannot
choose to enable threads in some cases.
This is breaking change to the CLI.
Related: #10143
40f5e5dfc6