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!
@alexcrichton I felt too insecure when I saw `Option::expect()`.  I'll use `.unwrap_or("unknown-target-triple")` instead.