[PATCH v4 1/2] rust: Refactor the build target to allow the use of builtin targets

[Alice Ryhl]

Jamie Cunliffe <Jamie.Cunliffe at arm.com> writes:
> Eventually we want all architectures to be using the target as defined
> by rustc. However currently some architectures can't do that and are
> using the target.json specification. This puts in place the foundation
> to allow the use of the builtin target definition or a target.json
> specification.
> 
> Signed-off-by: Jamie Cunliffe <Jamie.Cunliffe at arm.com>

Tested with Rust Binder.

Tested-by: Alice Ryhl <aliceryhl at google.com>