Andrew Kelley 40f5e5dfc6 CLI: introduce -fsingle-threaded/-fno-single-threaded
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
2021-11-30 19:21:29 -07:00
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