coff split: let the linker regenerate a PE export table

[handsomematt]

Adds a generate_exports module option: the linker builds the image's export directory instead of the split objects carrying the original's bytes. Needs openblack/llvm-project#4 for the vc6 layout.

What changes

• Directives: each split object gets a -export: directive for every symbol it defines that the image exports under that name.
  • Data exports get ,DATA.
  • An incrementally linked image exports through @ILT+n_ jump thunks, which become -export:name=@ILT+n_name.
  • The split fails if an export has no symbol of that name at its address. The fix there is a symbols.txt name or alias, for example for functions folded by link.exe's /OPT:ICF.
• Linker args: args.rsp gains /DEF:exports.def, whose LIBRARY line carries the original DLL name, and /TIMESTAMP: with the original directory's link time.
• Fold: coff fold keeps a recompiled unit's -export: directives for such a module. Without the option it still drops them, as in v0.0.33.
• Splits: they must leave the original table out of the linked units; bw1-decomp carves an .edata region and holds that unit back.

Testing

• cargo test: 32 pass, including new tests for the directives (plain, C name, @ILT thunk, ,DATA, a missing export), the .drectve that write_coff emits, and fold keeping exports.
• clippy has no diagnostics on the changed lines.
• bw1-decomp BW1W120 with a local build of lld: Lay out export and import tables like vc6 link.exe on i386 llvm-project#4:
  • LHAudio, LHLog, LHMultiplayer and LHDialog link their export tables from these directives;
  • all 15 build.sha1 hashes match.

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