compiletest: -Zunstable-options for json targets

[davidtwco]

bootstrap runs compiletest with synthetic custom targets when blessing mir-opt tests, but that no longer works after #150151/#151534 because -Zunstable-options is required

Contexts on Zulips.

[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu

[rustbot]

r? @Zalathar

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[SpriteOvO]

Tested on my machine, works fine now~

Maybe we should find time to add bless tests to CI, at least ensure it compiles so we don't miss it next time.

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