Implement the DT_GNU_HASH mechanism for vDSO parsing.

[sunfishcode]

Linux recently removed the DT_HASH section from the aarch64 vDSO. To continue to be able to read vDSOs, implement the DT_GNU_HASH section, following the logic in this patch.

[ids1024]

I wonder if vdso.rs could use less unsafe code. If the vDSO is just a region of memory that isn't mutated, it could be cast as slice (or portions of it could be; I guess ELF headers don't have a field that simply represents the size of the whole elf binary?). Something like #1253 should at least not be a segfault.

Though I guess if it mostly just matches the reference implementation in C, this would make it a bit less close.

[sunfishcode]

@ids1024 My sense is that keeping the code close to the reference implementation is more valuable in this situation. The ELF data structures are pretty subtle, and it's easy to accidentally use them in ways that work but aren't guaranteed by the ABI. By sticking super close to the reference code, we can be more confident that we're only depending on things that matter.