line_descriptor: enable __popcnt intrinsic for knn_match function

[pratham-mcw]

• This PR optimizes the popcnt implementation used by the line_descriptor module.
• Currently, ARM64 uses scalar implementation and this change enables the use of the hardware __popcnt intrinsic, while preserving the existing scalar implementation as a fallback.
• The optimization improves the performance of the knn_match function and also benefits several other functions in the line_descriptor module that rely on the same popcnt implementation.

Performance Benchmarks:

• 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