Update time_rtc example to use `time` crate

[ian-h-chamberlain]

Depends on https://github.com/Meziu/rust-linker-fix-3ds/pull/6

Couple of notes:

• This works as-is because the time crate does not enable its local-offset feature by default and therefore does not use libc::tm::tm_gmtoff, which is missing from the newlib definitions: https://github.com/rust-lang/libc/blob/master/src/unix/newlib/mod.rs#L105
• chrono seems like a slightly nicer crate to use, but it does support some timezone-awareness by default, and fails to build due to the aforementioned tm_gmtoff. Probably a PR upstream for chrono to support target_env = "newlib" more gracefully could resolve that, but it seemed a little beyond me...

For time itself:

• There is a formatting feature which allows a format like "[hour]:[minute]:[second]", but what I have now seems slightly more rust-idiomatic to me at least? I can enable that feature and use it if that seems preferable/
• I ripped out all the weekday, month code etc. since time provides that stuff itself.

[AzureMarker]

We could add libc::tm::tm_gmtoff to our libc fork, if we wanted.

[Meziu]

We could add libc::tm::tm_gmtoff to our libc fork, if we wanted.

No we can't. Newlib (at least devkitARM's 3DS fork) doesn't support it in it's functions. Sad, yeah, but it's normal when working with literally any unix target other than linux.