Cleaned up os::consts. The module only exposes constants for the target OS and arch.

Constants for other OS's and arch's must be defined manually.
[breaking-change]
auto merge of #13697 : pongad/rust/consts, r=alexcrichton

I decided to put architecture constants in another mod. They are not used, so a part of me is thinking of just getting rid of them altogether. The rest should be similar to what @brson wants.
Fixes #13536