csu: drop support for GCJ

[brooks]

Summary

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 csu files.

GCC removed support from trunk in on September 30, 2016 and the last
release with support with GCC 6.5. At this point we don't even have a
port capability of producing GCJ binaries.

For reference see:

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

[brooks]

This came in with D17587 for compatibility with old object files. It was roughly contemporaneous with GCC 6.5, the last release with GCJ support.

[emaste]

Sounds good to me.

[imp]

Change looks good to me. Might want to add the extra bit of history in your comment to the commit message... Seems important enough not to bury here.