Build fails with clang-cl: SVML _mm_pow_ps gated on _MSC_VER alone

`FixedFunctionOpCPU.cpp` gates the SVML `_mm_pow_ps` path on `_MSC_VER >= 1920` alone. clang-cl
defines `_MSC_VER` (1951 for clang 22.1.8) but ships no SVML, so the build breaks whenever
`OCIO_USE_AVX` is on.

Minimal:

```c++
#include
__m128 f(__m128 x, __m128 e) { return _mm_pow_ps(x, e); }
```

```
> clang-cl -c -arch:AVX t.cpp
t.cpp(2,39): error: use of undeclared identifier '_mm_pow_ps'
```

Excluding the compiler in the gate is enough, and clang-cl then takes the existing `ssePower()` path
like any other non-MSVC compiler:

```diff
-#if (_MSC_VER >= 1920) && (OCIO_USE_AVX)
+#if (_MSC_VER >= 1920) && (OCIO_USE_AVX) && !defined(__clang__)
```

main has 2 such sites; 2.5.2 has 4, the extra two being the dispatch arms in
`GetFixedFunctionCPURenderer`.

clang 22.1.8, x86_64-pc-windows-msvc. Reproduced against 2.5.2 and main.

Inizia in FixedFunctionOpCPU.cpp e individua i due gate del preprocessore SVML _mm_pow_ps che usano _MSC_VER e OCIO_USE_AVX. Riproduci il problema con clang-cl -c -arch:AVX, quindi verifica che la build clang-cl utilizzi il percorso ssePower() esistente e abbia successo con OCIO_USE_AVX abilitato.

Scritto dal modello di indicizzazione a partire dal testo della issue.