[libc++] Fix `gps_time` formatting and related tests (#181560)

- The Standard wording in https://eel.is/c++draft/time.format#13 is similar
  to TAI formatting in that it's equivalent to formatting a `sys_time`
  with a fixed offset. Leap seconds should not be considered.

- Tests need to be adjusted by adding the number of leap seconds between
  the GPS epoch and the tested date, which is 15s for 2010 and 18s for
  2019.

- The TAI and GPS tests using `meow_time<cr::duration<long, ...>>`
  should use `long long` because the offset swill overflow a 32-bit
  signed integer.

NOKEYCHECK=True
GitOrigin-RevId: a28755537b41f1d0683fcf53ef09832a2d61670a