Support for specifying the code model

The default code model is usually unsuitable for kernels,
so we add an option to specify which model we want.
auto merge of #15698 : Zoxc/rust/code-model, r=alexcrichton

The default code model is usually unsuitable for kernels,
so we add an option to specify which model we want.

Testing for this would be fragile and very architecture specific and is better left to LLVM.