Add an "inherits" key or similar to JSON target specs

[Aatch]

Looking at #32818, most of the target spec used there is just copying the existing i686-pc-windows-msvc target settings. This seems a little unnecessary, especially if we add to the options in the spec. A key in the JSON that indicates an existing target to use as a base seems like it would be useful.

Potential names:

• "inherits"
• "extends"
• "base"

[Mark-Simulacrum]

Would this require an RFC? Or is this something that just needs to be implemented?

[Mark-Simulacrum]

@eddyb I recall you mentioning that we wanted inheriting support (or had?); perhaps that's implemented today? I don't think so, but ... maybe?

[eddyb]

No idea. cc @rust-lang/compiler

[steveklabnik]

Triage: no changes.