fix Wasm exceptions + coop threading + shared libraries (#222747)

Prior to this commit, the combination of Wasm exception handling,
cooperative multithreading, and shared libraries was broken.
Specifically, the code generation in `WasmEHPrepare.cpp` involved
direct, cross-library access to `libunwind.so`'s thread-local
`__wasm_lpad_context` variable. However, the ABI used for cooperative
multithreading does not support cross-library access to thread-local
variables.

The solution used here is to add a new `_Unwind_GetWasmLPadContext`
function to `libunwind.so` and use that to get address of the
`__wasm_lpad_context` for the current thread, both in the code generated
by `WasmEHPrepare.cpp` and in the `__gxx_wasm_personality_v0` function
defined in `cxa_personality.cpp`. I've used this strategy
unconditionally for all targets, regardless of whether cooperative
multithreading and/or position-independent are enabled. If desired (e.g.
for performance or code complexity reasons), I could make it conditional
on both of those features being enabled and fall back to using
`__wasm_lpad_context` directly otherwise.

Co-authored-by: Derek Schuff <dschuff@chromium.org>
GitOrigin-RevId: 64907b6d3201f1755a149d029b3a65c4e0ee663f