Print the "file" command suggestion in exec_file_locate_attach

Print the "file" command suggestion in exec_file_locate_attach

currently:
	$ gdbserver-7.9 :1234 true &
	$ gdb -q -ex 'target remote :1234' # that -q is not relevant here
	Remote debugging using :1234
	warning: Could not load vsyscall page because no executable was specified
	try using the "file" command first.
	0x00007ffff7ddcc80 in ?? ()
	(gdb) b main
	No symbol table is loaded.  Use the "file" command.
	Make breakpoint pending on future shared library load? (y or [n]) _

Provide more suggestive message to use the "file" command.

gdb/ChangeLog
2016-04-06  Jan Kratochvil  <jan.kratochvil@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	* exec.c (exec_file_locate_attach): Print warning for unsupported
	target_pid_to_exec_file.
	* symfile-mem.c (add_vsyscall_page): Remove the "file" command
	message part.