Constant SINGLE_THREADED_TARGET

pub const SINGLE_THREADED_TARGET: bool = _; // false

Whether the target is single-threaded.

We still allow passing -j greater than 1, but it should gracefully handle the error when spawning the thread pool.

Modified from libtest: https://github.com/rust-lang/rust/blob/96cfc75584359ae7ad11cc45968059f29e7b44b7/library/test/src/lib.rs#L605-L607