These files are from libunwind, release 11.0.0.

tag: llvmorg-11.0.0
git: 176249bd6732a8044d457092ed932768724a6f06

Local Modification
------------------

Local modifications are marked with the comment: 'XXX EMSCRIPTEN'

The following changes are not marked with 'XXX EMSCRIPTEN'.

1. Add Unwind-wasm.cpp, a wasm implementation for libunwind.
