This was agreed on in
https://lists.llvm.org/pipermail/llvm-dev/2020-May/141345.html
The thread proposed two options
- add a character to augmentation string and handle in libuwind
- use a separate personality function.
It was determined that this is the simpler and better option.
This is part of ARM's Aarch64 ABI:
https://github.com/ARM-software/abi-aa/blob/main/aadwarf64/aadwarf64.rst#id22
The next step after this is teaching libunwind to untag when this
augmentation character is set.