Optimize Time.utc

Time.utc uses timegmw() and it uses leap second information.
If the given time is larger than known_leap_seconds_limit, it calls
find_time_t, which uses localtime(3) and calls stat(2) in it.

This patch avoid it by setting known_leap_seconds_limit to 0 if the timezone doesn't have leapsecond information (if no leap second is found "now", I assume the timezone doesn't have leapsecond information).

Before:
% time ./miniruby --disable-gem -e'time = Time.now; year = time.year; month = time.month; day = time.day; hour = time.hour; min = time.min; sec = time.sec + time.subsec; i = 0; while i < 100000; ::Time.utc(year, month, day, hour, min, sec); i += 1; end'
./miniruby --disable-gem   0.35s user 0.19s system 99% cpu 0.542 total

After:
% time ./miniruby --disable-gem -e'time = Time.now; year = time.year; month = time.month; day = time.day; hour = time.hour; min = time.min; sec = time.sec + time.subsec; i = 0; while i < 100000; ::Time.utc(year, month, day, hour, min, sec); i += 1; end'
./miniruby --disable-gem   0.23s user 0.00s system 99% cpu 0.233 total

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63848 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

time.c

@@ -1095,6 +1095,9 @@ init_leap_second_info(void)
         timew = timegmw_noleapsecond(&vtm);
 
         number_of_leap_seconds_known = NUM2INT(w2v(wsub(TIMET2WV(known_leap_seconds_limit), rb_time_unmagnify(timew))));
+        if (number_of_leap_seconds_known == 0) {
+            known_leap_seconds_limit = 0;
+        }
     }
 }