Fix v_signmask for RISC-V Vector

[hanliutong]

The original signmask implementation returned incorrect results in some cases (e.g. new test cases, 242 is returned but 2 is expected).

This is due to Vector Tail Agnostic in RVV, and this patch solves the problem by using "bitwise AND" to mask high bit.

Pull Request Readiness Checklist

See details at https://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

[asmorkalov]

@hanliutong thanks for the fix. Could you squash commits before megre?

[hanliutong]

@asmorkalov Commits are squashed now.