Merge pull request #25586 from hanliutong:rvv-64f

Fix v_round and enable unit tests for scalable universal intrinsic 64F type. #25586

This may be a legacy issue from the previous PR #24325. I don't quite remember why the float 64 part of the unit test was not enabled at that time.

Whatever, this patch enables the unit tests for scalable 64F type , and makes the necessary modifications to the RVV backend to make the tests pass.

This patch is compiled by GCC 14 and LLVM 17 &18, and tested on QEMU and k230.

### 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

modules/core/test/test_operations.cpp

@@ -1574,11 +1574,7 @@ TEST(Core_Arithm, scalar_handling_19599)  // https://github.com/opencv/opencv/is
 typedef tuple<perf::MatDepth,int,int,int> Arith_Regression24163Param;
 typedef testing::TestWithParam<Arith_Regression24163Param> Core_Arith_Regression24163;
 
-#if defined __riscv
-TEST_P(Core_Arith_Regression24163, DISABLED_test_for_ties_to_even)
-#else
 TEST_P(Core_Arith_Regression24163, test_for_ties_to_even)
-#endif
 {
     const int matDepth = get<0>(GetParam());
     const int matHeight= get<1>(GetParam());