All compilers should define this attribute. Probably should change base
class to require this.

Instead use coredata.compiler_options.<machine>. This brings the cross
and native code paths closer together, since both now use that.

Command line options are interpreted just as before, for backwards
compatibility. This does introduce some funny conditionals. In the
future, I'd like to change the interpretation of command line options so

 - The logic is cross-agnostic, i.e. there are no conditions affected by
   `is_cross_build()`.

 - Compiler args for both the build and host machines can always be
   controlled by the command line.

 - Compiler args for both machines can always be controlled separately.