Add regression test for PR gdb/19858 (JIT code registration on attach)

Add regression test for PR gdb/19858 (JIT code registration on attach)

This test would fail without the previous gdb/jit.c fix:

  (gdb) attach 23031
  Attaching to program: .../build/gdb/testsuite/outputs/gdb.base/jit/jit-main, process 23031
  [...]
  207           WAIT_FOR_GDB; i = 0;  /* gdb break here 1 */
  (gdb) PASS: gdb.base/jit.exp: attach: one_jit_test-2: attach
  set var wait_for_gdb = 0
  (gdb) PASS: gdb.base/jit.exp: attach: one_jit_test-2: set var wait_for_gdb = 0
  info function ^jit_function
  All functions matching regular expression "^jit_function":
  (gdb) FAIL: gdb.base/jit.exp: attach: one_jit_test-2: info function ^jit_function

gdb/testsuite/ChangeLog:
2016-03-31  Pedro Alves  <palves@redhat.com>

	PR gdb/19858
	* gdb.base/jit-main.c: Include unistd.h.
	(ATTACH): Define to 0 if not already defined.
	(wait_for_gdb, mypid): New globals.
	(WAIT_FOR_GDB): New macro.
	(MAIN): Set an alarm.  Store the process's pid.  Wait for GDB at
	some breakpoint locations.
	* gdb.base/jit.exp (clean_reattach, continue_to_test_location):
	New procedures.
	(one_jit_test): Add REATTACH parameter, and handle it.  Use
	continue_to_test_location.
	(top level): Test attach, and adjusts calls to one_jit_test.