Add basic support for libstd without a runtime.

To use it, pass `--cfg no_rt` to rustc when building libstd. Not everything is 
implemented correctly, and it probably needs to be reworked to use a 
zero.rs-like description of the runtime that *does* exist. Careful 
consideration and reworking needs to be done to make the libstd able to be run 
on bare metal.