cmake: fix generation for system name iOS

cmake: fix generation for system name iOS

This PR fixes a problem that happens during CMake configuration when
the `CMAKE_SYSTEM_NAME` set to `iOS` and not `Darwin`. This value is
available (as far as I remember) version 3.14. The final solution
(thanks to @vszakats) is to use `APPLE` which contains all the Apple
platforms https://cmake.org/cmake/help/latest/variable/APPLE.html.

This issue was found when during vcpkg installation. Running command
`vcpkg install curl:arm64-ios` and `vcpkg install curl:x64-ios` failed
with message:
```
CMake Error: try_run() invoked in cross-compiling mode, please set the following cache variables appropriately:
   HAVE_H_ERRNO_ASSIGNABLE_EXITCODE (advanced)
```
After this fix, I was able to compile the compile the binary without
any issue.

In addition to that fix, this PR also contains an simplification to
check if the platform is not APPLE.

Co-authored-by: Viktor Szakats
Closes #12515