swift234

[build-script] Do not determine build jobs in impl

Begins work on SR-237.

Rather than determining the number of jobs to use for `make` or
`dsymutil` from within build-script-impl, this change passed in those
values from above.

Furthermore, because we can determine the number of build jobs via a
builtin Python function, we figure out the number of jobs to use from
within `utils/build-script`. Now, by default, the same number of jobs
will be used to build this project as there are cores on the host
machine. Users may still specify a different number, as before, using
the `--jobs` option.

This introduces a behavioral change. Before, running
`utils/build-script` would run something like the following `cmake`
command:

```
cmake --build /build/Ninja-DebugAssert/llvm-macosx-x86_64 -- all
```

Notice the lack of jobs specified. The new default is something like
the following:

```
cmake --build /build/Ninja-DebugAssert/llvm-macosx-x86_64 -- -j8 all
```

Notice that the number of jobs is exlicitly passed to the command.