coff split: let the linker regenerate a PE import table

[handsomematt]

Adds a generate_imports module option: the linker builds the image's import directory instead of the split objects carrying the original's bytes. Needs openblack/llvm-project#4 for /vc6imports:.

What the split writes (read back from the original import directory)

• imports.lib: a COFF archive of short-format import objects, one per imported function.
  • Each carries the original hint and imported name (IMPORT_OBJECT_NAME_EXPORTAS), or the ordinal.
  • All are DATA imports, so the linker defines only __imp_<symbol>. The image's own jmp [__imp_X] stubs stay in the split objects, where link.exe put them.
• imports.order: the layout link.exe gave the tables. link.exe 6 ordered them by its library-search order, which can't be redone without the original import libraries:
  • descriptor order;
  • IAT/lookup slot order (grouped by DLL, not name-sorted within one);
  • the hint/name area with the DLL names interleaved;
  • pad lines for the zero padding an incremental link leaves after each DLL's entries, or that the splits' .idata$5 covers past the IAT.
• args.rsp: passes the library and /vc6imports:<order>.

Requirements

• The IAT slots must be named __imp_… in the symbols.
• The splits must leave the original .idata$N out of the linked units; bw1-decomp holds those units back.

Testing

• cargo test: 36 pass, including new tests for the import objects, the archive's symbol table and member offsets, the order file, and both kinds of padding.
• clippy has no diagnostics on the new code.
• bw1-decomp BW1W120 with a local build of lld: Lay out export and import tables like vc6 link.exe on i386 llvm-project#4:
  • runblack.exe, LHAudio, LHLog, LHMultiplayer and LHDialog (incrementally linked, with MFC ordinal imports) link their import tables from these;
  • all 15 build.sha1 hashes match.

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