[libunwind][WebAssembly] Support Wasm EH
This adds Wasm-specific libunwind port to support Wasm exception
handling (https://github.com/WebAssembly/exception-handling).
Wasm EH requires `__USING_WASM_EXCEPTIONS__` to be defined. This adds
`Unwind-wasm.c`, which defines libunwind APIs for Wasm. This also adds a
`thread_local` struct of type `_Unwind_LandingPadContext`, which serves
as a medium for input/output data between the user code and the
personality function. How all these work is explained in
https://github.com/WebAssembly/tool-conventions/blob/main/EHScheme.md.
(The doc is old and "You Shouldn't Prune Unreachable Resumes" section
doesn't apply anymore, but otherwise it should be good)
The bulk of these changes was added back in Mar 2020 in
https://github.com/emscripten-core/emscripten/pull/10577 to emscripten
repo and has been used ever since. Now we'd like to upstream this so
that other toolchains that don't use emscripten libraries, e.g., WASI,
can use this too.
Companion patch: D158918
Reviewed By: dschuff, #libunwind, phosek
Differential Revision: https://reviews.llvm.org/D158919
GitOrigin-RevId: 058222b2316615194c089f2bc68d11341f39d26e
2 files changed
