FROMLIST: rust: Refactor the build target to allow the use of builtin targets

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.

Bug: 308623796
Change-Id: I4e07024876dadad12be24ab5579b694e884eb638
Signed-off-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>
Link: https://lore.kernel.org/r/20231020155056.3495121-2-Jamie.Cunliffe@arm.com


Signed-off-by: Matthew Maurer <mmaurer@google.com>