gdb/features: set osabi in all Linux related features/*.xml files

gdb/features: set osabi in all Linux related features/*.xml files

Some of the top level (i.e. those that contain the <target> element)
xml files in gdb/features/ are clearly Linux only.  I conclude this
based on the files names containing the string "linux".

I think that all of these files should have the <osabi> element
included with the value "GNU/Linux".

This commits adds the <osabi> element where I believe it is
appropriate and regenerates the associated .c files.

The benefit of this change is that gdbserver, which makes use of these
files, will now send the osabi back in more cases.  Sending back more
descriptive target descriptions is a good thing as this makes it
easier for GDB to select the correct gdbarch.