Define libc++ config macros to nothing

libc++[abi] always check whether one of these macros is defined, not
the value the macro is defined to. With the new libc++, there is a
Windows __config_site header that automatically defines
_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS and _LIBCPP_HAS_THREAD_API_WIN32
to nothing, so these definitions need to match to avoid a redefinition
error.

Bug: 175635923
Test: treehugger
Test: m adb
Change-Id: I1e48947c3b45e59804cdacd48776c7f3bd9a18c6