Fix compilation with ClangCL and AVX2

[solbjorn]

Both __builtin_sadd_overflow() and __builtin_ctz() are available in Clang even when building under Visual Studio, define them only when compiling with MSVC to fix the following error:

opus\silk\x86\NSQ_del_dec_avx2.c(81,19): warning : redeclaring non-static '__builtin_ctz' as static is a Microsoft extension [-Wmicrosoft-redeclare-static]
     81 | static inline int __builtin_ctz(unsigned int x)
        |                   ^
  C:\Program Files\LLVM\lib\clang\22\include\ia32intrin.h(43,10): note: '__builtin_ctz' is a builtin with type 'int (unsigned int)'
     43 |   return __builtin_ctz((unsigned int)__A);
        |          ^
opus\silk\x86\NSQ_del_dec_avx2.c(81,19): error : definition of builtin function '__builtin_ctz'
     81 | static inline int __builtin_ctz(unsigned int x)
        |                   ^
  1 warning and 1 error generated.