Tighten remote check in gdb.base/argv0-symlink.exp

Tighten remote check in gdb.base/argv0-symlink.exp

Check for gdbserver instead of dejagnu remote.  Unlike what the
comment says, the test actually fails with target remote + gdbserver
(it does pass with extended-remote).  The result is:

 FAIL -> KFAIL with --target_board=native-gdbserver
 KPASS -> PASS with --target_board=native-extended-gdbserver

gdb/testsuite/ChangeLog:
2017-10-12  Pedro Alves  <palves@redhat.com>
	    Simon Marchi  <simon.marchi@polymtl.ca>

	* gdb.base/argv0-symlink.exp: kfail on remote gdbserver,
	instead of on dejagnu remote boards.