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