Define _LIBUNWIND_IS_NATIVE_ONLY when building LLVM libunwind. am: 4b1bd5d680 am: f458efcac7

Original change: https://android-review.googlesource.com/c/platform/external/libunwind_llvm/+/1537465

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I5e9b7cc67f5fc44b3f4661861a1215e3fad28f44