support TargetBuild FD compilation for sys tests [skip ci]

tests/sys/testsys.hxproj

@@ -39,7 +39,7 @@
     <hidden path="obj" />
   </hiddenPaths>
   <!-- Executed before build -->
-  <preBuildCommand>haxe compile.hxml</preBuildCommand>
+  <preBuildCommand>"$(CompilerPath)/haxe" compile$(TargetBuild).hxml</preBuildCommand>
   <!-- Executed after build -->
   <postBuildCommand alwaysRun="False" />
   <!-- Other project options -->