bug#26966: New Java bootstrap

[Ricardo Wurmus]

This patch series removes GCJ and bootstraps the JDK with Jikes.

Up to now our Java bootstrap with GCJ involved nasty compromises:

* GCJ depended on an iterative Java compiler in the form of a binary of
  ECJ (the Eclipse compiler for Java)

* The GCJ sources include pre-compiled Java .class files coming from
  the GNU Classpath project.

I only learned of the second of these problems when I expressed my
sadness about the removal of GCJ and Andrew Haley replied that GCJ
depended on binaries to a larger extent than I was aware of.

Recently Ludo brought up Jikes on #guix and while it first looked like a
much less capable compiler (only implementing a limited set of Java 1.5)
it turned out to be enough to get a first bootstrap JDK built.  After
some experimentation I managed to build a version of ECJ from source,
which eventually led the way to a properly bootstrapped JDK with IcedTea.

Note that some work is still needed to make it all fully reproducible,
but that’s not the primary goal of this patch set.

--
Ricardo

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