gsk/ngl/fp16.c: Implement runtime F16C detection on MSVC

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.