Summary

Fix build failure on macOS caused by passing-march=armv8-a to Apple Clang
when KleidiCV is enabled.

Root cause

KleidiCV propagates-march=armv8-a via its compile options. Apple Clang
does not recognize this CPU target and fails with
unknown target CPU 'armv8-a'.

Fix

Filter out the unsupported-march=armv8-a flag at the OpenCV integration
layer (hal/kleidicv/CMakeLists.txt) when building on macOS with Apple Clang,
while preserving the flag for other ARM toolchains.

Impact

• Fixes macOS (Apple Clang) builds with KleidiCV enabled
• No change to Linux / Android ARM builds
• Does not modify third-party KleidiCV sources

Fixes#28187

Pull Request Readiness Checklist

See details athttps://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake