Support reading AIX big archive format

[ecnelises]

The archive format on AIX is called 'big archive' (see definition at https://www.ibm.com/docs/en/aix/7.3?topic=formats-ar-file-format-big), which is quite different from BSD and GNU variants.

This PR will add support for reading it, as part of the efforts porting Rust toolchain onto AIX. (Writing facility was done by wrapping LLVM upstream code in rust-lang/ar_archive_writer#2)

Also thanks to contribution from @jsji.

[philipc]

Why did you remove the fixed header definition?

[ecnelises]

Why did you remove the fixed header definition?

It's rather an internal structure only used in file structure recognition part (ArchiveFile::parse). I thought it's enough to read the whole part as bytes and parse.

[philipc]

I think it's part of the file format, which is an external structure, and it's documented as such in the link you gave. I prefer if you add it back in please.

Also, please don't mark conversation as resolved. It's better if I mark them as resolved myself once I have reviewed.

[philipc]

Thanks! I'll do some style changes in a following PR.

[philipc]

Is there a reason you didn't use the member table at the end of the archive that is pointed to by the fl_memoff field? My concern that is by using ar_nxtmem, a malformed file could cause an infinite loop.

[ecnelises]

Ah, I did not think of such case when nxtmem is malformed (not a reasonable scenario to me at first). But yes, linked list may cause infinite loop. I'll try and post a new PR to iterate over the member table for offsets.