On Wasm, it's not currently possibly to change which `personality` is
attached to a given IR function. This is primarily because no matter
what `personality` is attached, codegen creates a call to a
`Unwind_CallPersonality` wrapper hard-coded to call c++abi's
personality.
In order to allow alternative personalities without introducing bloat at
each personality call site, this PR adjusts the ABI of the personality
on Wasm. `Unwind_CallPersonality` is moved to `libcxxabi` under the name
`__gxx_wasm_personality_v0`, with the "real" personality code renamed
`__gxx_personality_imp` and called by `__gxx_wasm_personality_v0` (after
resetting the landing pad context).
The signature of EH personalities on Wasm is now `int personality(void*
exception_object)` (where the int return is `_Unwind_Reason_Code` and
the `void*` is `_Unwind_Exception*`), and personalities are expected to
`__wasm_lpad_context.selector = 0;`.
Based on #175202
1d4e04f7a7