Apple's SoC's may be designed in house, but I'd bet the ARM cores in them are shared with Samsung. There's probably still a deal between Sammy and Intrinsity with regards to the low power trickery.
Apple bought Intrinsity in 2010.
Apple has no real reason to work on their own version of ARM on Samsung process when Samsung's done that already.
Arguably they have the most reason. They know where their OS is going so they know exactly what they need their CPU to do.
So far iPhone's cores always used whatever Samsung had on offer at the time, starting with stock Samsung SoCs, followed by a very slightly rehashed A4 that shared a great deal of similarity with Samsung's version through the A5.
The A4 and Hummingbird have a great deal of similarity given their shared Intrinsity heritage. However, A5 is a full custom design with quite a few differences from the Exynos 4 design. Besides having a completely different GPU core, the CPU cores and supporting circuitry are completely different sizes.
The point being - there's an A15 Exynos out now, it's practically given this is the basis of an A6 in the new iPhone.
It's inevitable they'll both use A15, that doesn't mean it's shared technology with the Exynos 5. We may be surprised and find out apple is taking a more custom approach a la Qualcomm's S4 Krait chip which is a sort of hybrid between A9 and A15, much like Snapdragon was a hybrid between A8 and A9.