Support "lstat" and "stat" on Linux (and all other Unixes)

[RalfJung]

This is like #4684, but for fn stat and fn lstat. #4714 did most of the work here, now all that's left is

• in the various foreign_items.rs files, make the same adjustments as that PR but for stat and lstat
• add tests directly calling stat and lstat to make sure it actually works