Enforce win64 abi test to always run on -m64 mode

Fixes #1439

gcc/testsuite/rust/debug/win64-abi.rs

@@ -1,10 +1,10 @@
+// { dg-do compile { target { x86_64-*-* } } }
+// { dg-options "-gdwarf-5 -dA -w -O1 -m64" }
 pub extern "win64" fn square(num: i32) -> i32 {
     num * num
 }
 
 fn main() {
-    // { dg-do compile { target { x86_64-*-* } } }
-    // { dg-options "-gdwarf-5 -dA -w -O1" }
     // MS ABI dictates that the first argument is ecx instead of edi from the sysv world
     // { dg-final { scan-assembler "%ecx, %ecx" } }
     square(1);