Select the correct target STL with USE_MINGW.

The previous check tested against USE_MINGW even for target builds.
Hadn't been a problem because people don't typically set USE_MINGW
directly.

Change-Id: I90fe0ea890c44917eb29dd02d7c7f76c19e7fbd6
diff --git a/core/cxx_stl_setup.mk b/core/cxx_stl_setup.mk
index da84bdd..cbabc35 100644
--- a/core/cxx_stl_setup.mk
+++ b/core/cxx_stl_setup.mk
@@ -8,16 +8,16 @@
 ifeq ($(strip $(LOCAL_CXX_STL)),default)
     ifndef LOCAL_SDK_VERSION
         # Platform code. Select the appropriate STL.
-        ifndef USE_MINGW
-            my_cxx_stl := libc++
-            ifdef LOCAL_IS_HOST_MODULE
-                ifneq (,$(BUILD_HOST_static))
-                    my_cxx_stl := libc++_static
-                endif
+        my_cxx_stl := libc++
+        ifdef LOCAL_IS_HOST_MODULE
+            ifneq (,$(BUILD_HOST_static))
+                my_cxx_stl := libc++_static
             endif
-        else
-            # libc++ is not supported on mingw.
-            my_cxx_stl := libstdc++
+
+            ifdef USE_MINGW
+                # libc++ is not supported on mingw.
+                my_cxx_stl := libstdc++
+            endif
         endif
     else
         my_cxx_stl := ndk