Add stubs and headers for nl_types APIs.

We don't have message catalog APIs on Android until O. For releases
prior to that, we build in stub APIs (as weak symbols so we use libc's
on new API levels). We add these here instead of in libandroid_support
because the driver (correctly) links libomp well after
libandroid_support, which means libandroid_support won't be searched
for resolving symbols in openmp.

Since NDK r16 removed nl_types.h, also bundle a stub header.

aosp/571945

Test: Build LLVM with NDK r16
Bug: https://github.com/android-ndk/ndk/issues/9
Change-Id: I0a07a969388b958be49bd50325bd7c5254140127