Auto merge of #86062 - nagisa:nagisa/what-a-lie, r=estebank

Do not allow JSON targets to set is-builtin: true

Note that this will affect (and make builds fail for) all of the projects out there that have target files invalid in this way. Crater, however, does not really cover these kinds of the codebases, so it is quite difficult to measure the impact. That said, the target files invalid in this way can start causing build failures each time LLVM is upgraded, anyway, so it is probably a good opportunity to disallow this property, entirely.

Another approach considered was to simply not parse this field anymore, which would avoid making the builds explicitly fail, but it wasn't clear to me if `is-builtin` was always set unintentionally… In case this was the case, I'd expect people to file a feature request stating specifically for what purpose they were using `is-builtin`.

Fixes #86017

diff --cc compiler/rustc_codegen_llvm/src/context.rs
Simple merge

diff --cc compiler/rustc_target/src/spec/mod.rs
Simple merge