Update cflags for building Windows modules with Clang

Bug: http://b/69933068

- Add -Wexit-time-destructor globally after fixing one warning
- Enable -Wthread-safety only for non-windows targets.  The version of
libstdc++ we use for Windows doesn't have the correct thread-safety
annotations.

Test: Run Clang-built adb_test.exe under wine
Change-Id: I76a43060342e7d1dc370ce3e117f3563c82b7751

adb/Android.bp

@@ -19,17 +19,13 @@ cc_defaults {
         "-Wall",
         "-Wextra",
         "-Werror",
+        "-Wexit-time-destructors",
         "-Wno-unused-parameter",
         "-Wno-missing-field-initializers",
         "-Wvla",
     ],
     rtti: true,
 
-    clang_cflags: [
-        "-Wexit-time-destructors",
-        "-Wthread-safety",
-    ],
-
     use_version_lib: true,
 
     compile_multilib: "first",
@@ -85,6 +81,12 @@ cc_defaults {
                 "-luserenv",
             ],
         },
+
+        not_windows: {
+            cflags: [
+                "-Wthread-safety",
+            ],
+        },
     },
 }