csu: drop support for GCJ

Remove .jcr sections and related infrastructure. This has no impact on
existing binaries and the toolchain remains capability of linking them,
but would require different (or additional) csu files.

GCC removed GCJ from trunk in on September 30, 2016. Our support came
in with D17587 for compatibility with old object files. It was roughly
contemporaneous with GCC 6.5, the last release with GCJ support. At
this point we don't even have a compiler port capable of producing GCJ
binaries so there is no need to carry this around any more.

For reference see:

https://en.wikipedia.org/wiki/GNU_Compiler_for_Java#History

Reviewed by: kib, emaste
Discussed with: imp
Sponsored by: SRI International
Differential Revision: https://reviews.freebsd.org/D49882