Enable gdb.threads/non-ldr-exc-*.exp on is_remote target boards

Enable gdb.threads/non-ldr-exc-*.exp on is_remote target boards

This commit makes the gdb.threads/non-ldr-exc-*.exp tests run (and
pass) with --target_board=native-gdbserver.

(These tests were already running with
--target_board=native-extended-gdbserver, because that board is not
is_remote.)

The "No exec event support in the remote protocol." comment is stale.
It's actually supported.

gdb/ChangeLog:
2017-10-12  Pedro Alves  <palves@redhat.com>

	* gdb.threads/non-ldr-exc-1.exp: No longer skip if is_remote target.
	* gdb.threads/non-ldr-exc-2.exp: Ditto.
	* gdb.threads/non-ldr-exc-3.exp: Ditto.
	* gdb.threads/non-ldr-exc-4.exp: Ditto.