Adding Support for Linux/390

[Post, Mark K]

When I went to upgrade to Guile 1.6.3, I discovered that it wouldn't compile
due to the addition of libguile/gc_os_dep.c.  I'm attaching a patch to add
support for Linux/390.  Only about 5 of the lines are mine.  I stole the
rest from other, previously existing, Linux entries.  Since this is part of
the garbage collector, I wasn't sure if this should go to Guile or them, but
I figured I would try you first.  Let me know if you need anything else.

Thanks,

Mark Post

gc_os_dep.c.diff
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