Check HXCPP_ARCH before defaulting to host arch

Right now HXCPP_ARCH is ignored, which means you can get situations
where `HXCPP_ARM64` is set but the architecture is x86_64 so you may end
up with a broken build.

tools/hxcpp/BuildTool.hx

@@ -136,12 +136,20 @@ class BuildTool
       arm64 = mDefines.exists("HXCPP_ARM64");
       if (m64==m32 && !arm64)
       {
-         var arch = getArch();
+         var arch = mDefines.get("HXCPP_ARCH");
+         if (arch != null) {
+            m64 = arch == "x86_64";
+            m32 = arch == "x86";
+            arm64 = arch == "arm64";
+         } else {
+            var hostArch = getArch();
+
+            // Default to the current OS version.  windowsArm runs m32 code too
+            m64 = hostArch == "m64";
+            m32 = hostArch == "m32";
+            arm64 = hostArch == "arm64";
+         }
 
-         // Default to the current OS version.  windowsArm runs m32 code too
-         m64 = arch=="m64";
-         m32 = arch=="m32";
-         arm64 = arch=="arm64";
          mDefines.remove(m32 ? "HXCPP_M64" : "HXCPP_M32");
          set64(mDefines,m64,arm64);
       }