librustc/driver: expose build information of rustc.

CFG_RELEASE, CFG_VER_HASH and CFG_VER_DATE were only available
as an output to stdout from the driver::version() function
that had an inconvenient signature.

Also, if one failed to set an env var CFG_COMPILER_HOST_TRIPLE at
compile time, the compilation fails.

Now we can easily check the exact version of rustc running at playpen.

I would appreciate if someone could improve my English in the rustdoc comments!
It's ok to fail at runtime (there's no good default for this), it's just there with an option to make building by hand a little easier.