Skip a few tests on targets that can't use the "run" commmand.

These tests want to use raw "run", so skip them on targets that can't
do that.

Also adds a small utility procedure that clearly conveys intent instead of
explicitly checking use_gdb_stub in the testcases.

This makes sure these testcases continue to be skipped with
--target_board=native-gdbserver once that board stops setting
is_remote.

gdb/testsuite/ChangeLog:
2017-10-13  Pedro Alves  <palves@redhat.com>

	* lib/gdb.exp (target_can_use_run_cmd): New procedure.
	* gdb.base/annota1.exp: Use it instead of is_remote.
	* gdb.base/annota3.exp: Use it instead of is_remote.
	* gdb.cp/annota2.exp: Use it instead of is_remote.
	* gdb.cp/annota3.exp: Use it instead of is_remote.
	* gdb.multi/bkpt-multi-exec.exp: Use it instead of is_remote.