Use TIME_T_MAX and TIME_T_MIN in tst-mktime2.c

GCC 5.3 compiles

for (time_t_max = 1; 0 < time_t_max; time_t_max *= 2)
    continue;

into an infinite loop with -Os.  We can copy TIME_T_MAX and TIME_T_MIN
from time/mktime.c.

	[BZ #19466]
	* time/tst-mktime2.c (time_t_max): Removed.
	(time_t_min): Likewise.
	(TYPE_SIGNED): New.
	(TYPE_MINIMUM): Likewise.
	(TYPE_MAXIMUM): Likewise.
	(TIME_T_MIN): Likewise.
	(TIME_T_MAX): Likewise.
	(mktime_test): Replace time_t_max and time_t_min with TIME_T_MAX
	and TIME_T_MIN.
	(do_test): Likewise.