[mlir][sparse] Fix the calling convention of __truncsfbf2 on windows x64

It also wants us to return the value in XMM0.

Author: d0k

mlir/lib/ExecutionEngine/Float16bits.cpp

@@ -165,7 +165,7 @@ bool operator==(const bf16 &f1, const bf16 &f2) { return f1.bits == f2.bits; }
 #endif
 #endif
 
-#if defined(__x86_64__)
+#if defined(__x86_64__) || defined(_M_X64)
 // On x86 bfloat16 is passed in SSE registers. Since both float and __bf16
 // are passed in the same register we can use the wider type and careful casting
 // to conform to x86_64 psABI. This only works with the assumption that we're