libbb: in @SECONDS date format, use 64-bit time if libc allows

function                                             old     new   delta
packed_usage                                       33472   33486     +14
parse_datestr                                        919     916      -3

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

coreutils/date.c

@@ -111,6 +111,7 @@
 //usage:	)
 //usage:     "\n"
 //usage:     "\nRecognized TIME formats:"
+//usage:     "\n	@seconds_since_1970"
 //usage:     "\n	hh:mm[:ss]"
 //usage:     "\n	[YYYY.]MM.DD-hh:mm[:ss]"
 //usage:     "\n	YYYY-MM-DD hh:mm[:ss]"