time: move C backend specific code into .c.v files (#11125)

vlib/time/chrono.c.v

@@ -0,0 +1,18 @@
+module time
+
+// portable_timegm does the same as C._mkgmtime, but unlike it,
+// can work with dates before the Unix epoch of 1970-01-01 .
+pub fn portable_timegm(t &C.tm) i64 {
+	mut year := t.tm_year + 1900
+	mut month := t.tm_mon // 0-11
+	if month > 11 {
+		year += month / 12
+		month %= 12
+	} else if month < 0 {
+		years_diff := (11 - month) / 12
+		year -= years_diff
+		month += 12 * years_diff
+	}
+	days_since_1970 := i64(days_from_civil(year, month + 1, t.tm_mday))
+	return 60 * (60 * (24 * days_since_1970 + t.tm_hour) + t.tm_min) + t.tm_sec
+}