checkpoint.exp: Check for non-"target native" instead of isnative/is_remote

checkpoint.exp: Check for non-"target native" instead of isnative/is_remote

This gets rid of a number of FAILs with
--target_board=native-extended-gdbserver.

The fact that checkpointing does not work has nothing to do with
dejagnu's native and remote concepts.  It only works with native Linux
targets because the implementation is currently baked with
linux-nat.c.

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

	* gdb.base/checkpoint.exp: Don't check is_remote or isnative.
	Instead skip if there's any gdb_protocol set.