hadrian/bootstrap/bootstrap.py: Add an option --jobs=N for parallel compilation

We at pkgsrc build GHC with an assumption that cabal-install is not available until we build GHC, cabal-install, and all of its dependencies. This means we need to regularly build Hadrian with a bootstrap source tarball. One of the problems is that bootstrap.py does not support parallel compilation and is therefore a slow process.

This patch adds an option --jobs=N (and -j N) to bootstrap.py so that we can bootstrap Hadrian parallelly.

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