Fix gdb.base/remote.exp with native-extended-gdbserver board

Fix gdb.base/remote.exp with native-extended-gdbserver board

This fixes gdb.base/remote.exp regressions caused by the previous
commit to the testcase, when tested with
--target_board=native-extended-gdbserver.  For example:

  ...
  show remote memory-write-packet-size
  The memory-write-packet-size is 0 (default). Packets are limited to 16383 bytes.
  (gdb) FAIL: gdb.base/remote.exp: write-packet default
  ...

With that board, GDB connects to GDBserver at gdb_start time, so GDB
is showing the actual remote/gdbserver packet size limits.

Fix it using the usual "disconnect" pattern.  While at it, there's no
need to start GDB before compiling the testcase.

gdb/testsuite/ChangeLog:
2018-05-22  Pedro Alves  <palves@redhat.com>

	* gdb.base/remote.exp: Only gdb_start after compiling the
	testcase.  Issue "disconnect" before testing "set remote" command
	defaults.  Issue clean_restart before running to main.