Merge "Switch libcompiler_rt to the LLVM prebuilt libunwind" am: 1eda370e3c

Original change: https://android-review.googlesource.com/c/platform/external/compiler-rt/+/1554717

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I7f70a5460b2f764b6a44bafb539cfb3bc4c4d880