Fix GDBHISTSIZE test failure on i686

Fix GDBHISTSIZE test failure on i686

The test

  test_histsize_history_setting "99999999999999999999999999999999999" "unlimited"

was failing on i686 because the condition in init_history() for
determining whether to map a large GDBHISTSIZE value to infinity was

  long var = strtol (tmpenv);
  if (var > INT_MAX)
    history_size = unlimited;

but this condition is never true on i686 because INT_MAX == LONG_MAX.
So in order to properly map large out-of-range values of GDBHISTSIZE to
infinity on targets where LONG_MAX > INT_MAX as well as on i686, we have
to instead change the above condition to

  if (var > INT_MAX
      || (var == INT_MAX && errno == ERANGE))
    history_size = unlimited;

gdb/ChangeLog:

	* top.c (init_history): Look at errno after calling strtol to
	properly map large GDBHISTSIZE values to infinity.