Merge pull request #27337 from fengyuentau:4x/build/riscv/fix_warnings

build: fix warnings from recent gcc versions #27337

This PR addresses the following found warnings:
- [x] -Wmaybe-uninitialized
- [x] -Wunused-variable
- [x] -Wsign-compare

Tested building with GCC 14.2 (RISC-V 64).

### Pull Request Readiness Checklist

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

- [x] I agree to contribute to the project under Apache 2 License.
- [x] 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
- [x] 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_intrin_utils.hpp

@@ -24,6 +24,17 @@ void test_hal_intrin_float16();
 
 //==================================================================================================
 
+#if defined (__GNUC__) && defined(__has_warning)
+    #if __has_warning("-Wmaybe-uninitialized")
+        #define CV_DISABLE_GCC_MAYBE_UNINITIALIZED_WARNINGS
+    #endif
+#endif
+
+#if defined (CV_DISABLE_GCC_MAYBE_UNINITIALIZED_WARNINGS)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+
 template <typename R> struct Data
 {
     typedef typename VTraits<R>::lane_type LaneType;
@@ -2409,6 +2420,10 @@ void test_hal_intrin_float16()
 }
 #endif*/
 
+#if defined (CV_DISABLE_GCC_MAYBE_UNINITIALIZED_WARNINGS)
+#pragma GCC diagnostic pop
+#endif
+
 #endif //CV_CPU_OPTIMIZATION_DECLARATIONS_ONLY
 
 //CV_CPU_OPTIMIZATION_NAMESPACE_END