Run rustdoc for target triple

[nbdd0121]

Currently we run rustdoc with host triple. This works now because kernel crate does not re-export items from other crates.

If I try to add pub use build_assert::{build_assert, build_error} in #257, make rustdoc will generate:

error[E0461]: couldn't find crate `build_assert` with expected target triple x86_64-unknown-linux-gnu
  --> rust/kernel/prelude.rs:22:9
   |
22 | pub use build_assert::{build_error, build_assert};
   |         ^^^^^^^^^^^^
   |
   = note: the following crate versions were found:
           crate `build_assert`, target triple target-10667598549189129223: /home/gary/rust-for-linux/rust/libbuild_assert.rmeta