Don't build multc3 on AArch64 MSVC

Apparently it doesn't compile due to an odd compiler error! This intrinsic has
to do with complex arithmetic anyway and we shouldn't need it.

build.rs

@@ -415,11 +415,14 @@ mod c {
                     "floatsitf.c",
                     "floatunditf.c",
                     "floatunsitf.c",
-                    "multc3.c",
                     "trunctfdf2.c",
                     "trunctfsf2.c",
                 ],
             );
+
+            if target_os != "windows" {
+                sources.extend(&["multc3.c"]);
+            }
         }
 
         // Remove the assembly implementations that won't compile for the target