gsk/ngl/fp16.c: Implement runtime F16C detection on MSVC
We need to use __cpuid() to check for the presence of F16C instructions on Visual Studio builds, and call the half_to_float4() or float_to_half4() implementation accordingly, as the __builtin_cpu...() functions are strictly for GCC or CLang only. Also, since __m128i_u is not a standard intrisics type across the board, just use __m128i on Visual Studio as it is safe to do so there for use for _mm_loadl_epi64(). Like running on Darwin, we cannot use the alias __attribute__ as __attribute__ is also for GCC and CLang only.
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