Change another _WIN32 to _MSC_VER for MinGW

Older MinGW versions do not know about __assume().
Change _WIN32 in comments too to match their opening #if.

Reported by serpentine.

idlib/math/Simd_3DNow.cpp

@@ -290,4 +290,4 @@ $memcpy_final:
 	}
 }
 
-#endif /* _WIN32 */
+#endif /* _MSC_VER */

idlib/math/Simd_Generic.cpp

@@ -49,7 +49,7 @@ If you have questions concerning this license or the applicable additional terms
 
 #ifdef _DEBUG
 #define NODEFAULT	default: assert( 0 )
-#elif _WIN32
+#elif _MSC_VER
 #define NODEFAULT	default: __assume( 0 )
 #else
 #define NODEFAULT

idlib/math/Simd_SSE2.cpp

@@ -872,4 +872,4 @@ void VPCALL idSIMD_SSE2::MixedSoundToSamples( short *samples, const float *mixBu
 	}
 }
 
-#endif /* _WIN32 */
+#endif /* _MSC_VER */

idlib/math/Simd_SSE3.cpp

@@ -362,4 +362,4 @@ void VPCALL idSIMD_SSE3::TransformVerts( idDrawVert *verts, const int numVerts,
 #endif
 }
 
-#endif /* _WIN32 */
+#endif /* _MSC_VER */