Double-word align _Unwind_Exception

[hsanzg]

The Itanium C++ ABI spec states that the _Unwind_Exception type must be double-word aligned (see Section 1.2). The missing alignment option does not seem to cause problems when a) using Rust's panic mechanism (because the exception object passed to _Unwind_RaiseException is heap-allocated and thus double-word aligned by accident---although this is not guaranteed) or b) when linking against libgcc's unwinder.

Most people don't need to unwind stacks themselves, so case (a) usually applies; and case (b) is the default for the amd64-unknown-linux-gnu target. Thus, misalignments seem unlikely to cause trouble in practice. However, I recently copied over some of the type definitions in library/unwind into a personal project, built rustc with rust.llvm-libunwind = "system", and installed LLVM's libunwind-24-dev. Calling _Unwind_RaiseException with a thread-local _Unwind_Exception object led to a segfault due to the too-small default alignment. So libunwind seems to depend on the double-word alignment.

N.B.: Both libunwind and libgcc have comments [1, 2] saying that the "double-word" alignment is a bit ambiguous when interpreted in a target-agnostic sense, and they both add __attribute__((__aligned__)) to _Unwind_Exception, with no specific alignment value (the default is the maximum alignment of any integer type). Rust doesn't have such a "default" alignment, so I interpreted "double-word" in a target-dependent manner via cfg_attr + the target_pointer_width feature.

[1]: https://github.com/llvm/llvm-project/blob/196786fa5fe4225539678fc7904a383eca05374e/libunwind/include/unwind_itanium.h#L41
[2]: https://github.com/gcc-mirror/gcc/blob/50a2eb56b9a350ecced4db4942e92d463dab8d8f/libgcc/unwind-generic.h#L106

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[jieyouxu]

r? libs

[tgross35]

Could you add a comment similar to what's in LLVM? Otherwise LGTM.

View changes since the review

[tgross35]

Cc @bjorn3 since this is your area of expertise