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Rollup merge of #152236 - davidtwco:compiletest-destabilise-custom-targets, r=scottmcm
compiletest: `-Zunstable-options` for json targets
bootstrap runs compiletest with synthetic custom targets when blessing `mir-opt` tests, but that no longer works after rust-lang/rust#150151/rust-lang/rust#151534 because `-Zunstable-options` is required
Contexts on [Zulips](https://rust-lang.zulipchat.com/#narrow/channel/122651-general/topic/custom.20targets.20are.20unstable.20and.20require.20.60-Zunstable-options.60).
