[gdb/testsuite] Handle host cleanfiles

[gdb/testsuite] Handle host cleanfiles

When running test-case gdb.server/abspath.exp with host board
local-remote-host-notty, I get:
...
$ git sti
  ...
        deleted:    gdb/testsuite/gdb.xml/trivial.xml
...

This happens as follows.  The test-case calls skip_gdbserver_test, which calls
gdb_skip_xml_test, which does:
...
    set xml_file [gdb_remote_download host "${srcdir}/gdb.xml/trivial.xml"]
...

Then proc gdb_remote_download appends $xml_file (which for this particular
host board happens to be ${srcdir}/gdb.xml/trivial.xml) to cleanfiles, which
ends up being handled in gdb_finish by:
...
       eval remote_file target delete $cleanfiles
...

The problem is that a host file is deleted using target delete.

Fix this by splitting cleanfiles up in cleanfiles_target and cleanfiles_host.

Tested on x86_64-linux.