intrinsic-test: run rustfmt on generated sources

[davidtwco]

This allows for simplification of the intrinsic-test generator as it no longer needs to maintain readable formatting.

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[sayantn]

Can we maybe do this in the intrinsic-test output, like gen-arm does?

[davidtwco]

Can we maybe do this in the intrinsic-test output, like gen-arm does?

Sure, I've changed it to work that way