boring-sys: Don't use CMake cross-compilation for macOS->iOS

[jrose-signal]

(or macOS->macOS)

Fixes the iOS build.

[nox]

Thanks for the PR. Could you explain what it fixes? Maybe show an example of a broken build with the current code?

[jrose-signal]

You can see one example in this log after fixing the CI to test cross-compilation more thoroughly (#188). In this case the assembly sources end up getting built for iOS devices rather than iOS simulators, presumably because CMake assumes you'll set all of the relevant cross-compilation properties if you're going to use any of them. But for toolchains that natively support multiple targets, this just makes things worse.

(Honestly, I don't think this sort of policy should be happening here at all; it should be in the cmake crate. But that has similar problems, and since the cmake crate isn't being actively developed I understand doing things here!)

[jrose-signal]

Citation from the CMake docs themselves:

Another case to be aware of is that builds targeting Apple platforms other than macOS are handled differently to other cross compiling scenarios. Rather than relying on CMAKE_SYSTEM_NAME to select the target platform, Apple device builds use CMAKE_OSX_SYSROOT to select the appropriate SDK, which indirectly determines the target platform. […] Therefore, the use of CMAKE_CROSSCOMPILING is not recommended for projects targeting Apple devices.

[nox]

Sorry I had other yaks to shave these last few days but I'll review these tomorrow and so far I don't see a reason not to merge those changes.

[jrose-signal]

No worries, that was less a "hey, pay attention to me" and more "I found a citation to support this, let me put in the PR for posterity"!

[nox]

I fixed the failing clippy lint in a separate PR so you can just rebase yours for CI to go green.