Merge pull request #27006 from hanliutong:rvv-fix-ui-1024

Fix issues in RISC-V Vector (RVV) Universal Intrinsic #27006

This PR aims to make `opencv_test_core` pass on RVV, via following two parts:

1. Fix bug in Universal Intrinsic when VLEN >= 512:
- `max_nlanes` should be multiplied by 2, because we use LMUL=2 in RVV Universal Intrinsic since #26318.
- Related tests are also expanded to match longer registers
- Relax the precision threshold of `v_erf` to make the tests pass

2. Temporary fix  #26936
- Disable 3 Universal Intrinsic code blocks on GCC
- This is just a temporary fix until we figure out if it's our issue or GCC/something else's

This patch is tested under the following conditions:
- Compier: GCC 14.2, Clang 19.1.7
- Device: Muse-Pi (VLEN=256), QEMU (VLEN=512, 1024)


### 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
- [ ] 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/src/convert.simd.hpp

@@ -108,7 +108,8 @@ cvt_( const _Ts* src, size_t sstep, _Td* dst, size_t dstep, Size size )
     for( int i = 0; i < size.height; i++, src += sstep, dst += dstep )
     {
         int j = 0;
-#if (CV_SIMD || CV_SIMD_SCALABLE)
+// Excluding GNU in CV_SIMD_SCALABLE because of "opencv/issues/26936"
+#if (CV_SIMD || (CV_SIMD_SCALABLE && !(defined(__GNUC__) && !defined(__clang__))) )
         const int VECSZ = VTraits<_Twvec>::vlanes()*2;
         for( ; j < size.width; j += VECSZ )
         {

modules/core/src/convert_scale.simd.hpp

@@ -92,7 +92,7 @@ template<typename _Ts, typename _Td> inline void
 cvt_32f( const _Ts* src, size_t sstep, _Td* dst, size_t dstep,
          Size size, float a, float b )
 {
-#if (CV_SIMD || CV_SIMD_SCALABLE)
+#if (CV_SIMD || (CV_SIMD_SCALABLE && !(defined(__GNUC__) && !defined(__clang__))) )
     v_float32 va = vx_setall_f32(a), vb = vx_setall_f32(b);
     const int VECSZ = VTraits<v_float32>::vlanes()*2;
 #endif
@@ -102,7 +102,8 @@ cvt_32f( const _Ts* src, size_t sstep, _Td* dst, size_t dstep,
     for( int i = 0; i < size.height; i++, src += sstep, dst += dstep )
     {
         int j = 0;
-#if (CV_SIMD || CV_SIMD_SCALABLE)
+// Excluding GNU in CV_SIMD_SCALABLE because of "opencv/issues/26936"
+#if (CV_SIMD || (CV_SIMD_SCALABLE && !(defined(__GNUC__) && !defined(__clang__))) )
         for( ; j < size.width; j += VECSZ )
         {
             if( j > size.width - VECSZ )
@@ -163,7 +164,7 @@ template<typename _Ts, typename _Td> inline void
 cvt_64f( const _Ts* src, size_t sstep, _Td* dst, size_t dstep,
          Size size, double a, double b )
 {
-#if (CV_SIMD_64F || CV_SIMD_SCALABLE_64F)
+#if (CV_SIMD_64F || (CV_SIMD_SCALABLE_64F && !(defined(__GNUC__) && !defined(__clang__))) )
     v_float64 va = vx_setall_f64(a), vb = vx_setall_f64(b);
     const int VECSZ = VTraits<v_float64>::vlanes()*2;
 #endif
@@ -173,7 +174,8 @@ cvt_64f( const _Ts* src, size_t sstep, _Td* dst, size_t dstep,
     for( int i = 0; i < size.height; i++, src += sstep, dst += dstep )
     {
         int j = 0;
-#if (CV_SIMD_64F || CV_SIMD_SCALABLE_64F)
+// Excluding GNU in CV_SIMD_SCALABLE because of "opencv/issues/26936"
+#if (CV_SIMD_64F || (CV_SIMD_SCALABLE_64F && !(defined(__GNUC__) && !defined(__clang__))) )
         for( ; j < size.width; j += VECSZ )
         {
             if( j > size.width - VECSZ )