[llvm-dlltool] Set a proper machine type for weak symbol object files

[mstorsjo]

Summary

This makes GNU binutils not reject the libraries outright.

GNU ld handles weak externals slightly differently though, so it can't use them for aliases in import libraries, but this makes GNU ld able to use the rest of the import libraries.

LLD hasn't had issues with the bogus machine field though.

[ruiu]

LGTM

[mstorsjo]

@tstellard - This would be nice to have in 8.0.1 as well.

[mstorsjo]

(Sorry for the double post due to a typo) @tstellar - This would be nice to have in 8.0.1 as well.

lib/Object/COFFImportFile.cpp

 NewArchiveMember ObjectFactory::createWeakExternal(StringRef Sym,
                                                    StringRef Weak, bool Imp) {
   std::vector<uint8_t> Buffer;
   const uint32_t NumberOfSections = 1;
   const uint32_t NumberOfSymbols = 5;
 
   // COFF Header
   coff_file_header Header{
-      u16(0),
+      u16(Machine),
       u16(NumberOfSections),
       u32(0),
       u32(sizeof(Header) + (NumberOfSections * sizeof(coff_section))),
       u32(NumberOfSymbols),
       u16(0),
       u16(0),
   };
   append(Buffer, Header);

test/tools/llvm-dlltool/coff-weak-exports.def

 ; RUN: llvm-dlltool -m i386:x86-64 --input-def %s --output-lib %t.a
 ; RUN: llvm-nm %t.a | FileCheck %s
+; RUN: llvm-readobj %t.a | FileCheck -check-prefix=ARCH %s
 
 LIBRARY test.dll
 EXPORTS
 TestFunction==AltTestFunction
 ; When creating an import library, the DLL internal function name of
 ; the implementation of a function isn't visible at all.
 ImpLibName = Implementation
 ; A different import library name and implementation name can be mixed

 ; CHECK:      T ImpLibName
 ; CHECK-NEXT: T __imp_ImpLibName
 ; CHECK:      U AltTestFunction2
 ; CHECK-NEXT: W ImpLibName2
 ; CHECK:      U __imp_AltTestFunction2
 ; CHECK-NEXT: W __imp_ImpLibName2
 ; CHECK:      T ImpLibName3
 ; CHECK-NEXT: T __imp_ImpLibName3
+
+; ARCH-NOT: unknown arch