Define _LIBUNWIND_IS_NATIVE_ONLY when building LLVM libunwind. am: 4b1bd5d680

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

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I2eff3c70e9c2141197be990c64659d1ec7b2b89d