Add -linker command-line option

[kinke]

Needed for LTO via external lld-link.exe (v5.0+).

[kinke]

We quickly hit an LLVM 5.0 unreachable() when trying to compile druntime (or Phobos) with -flto=full on Windows:

this directive only supported on COFF targets
UNREACHABLE executed at C:\LDC\llvm\lib\MC\MCStreamer.cpp:811!
#4 0x00007ff6dcded49d llvm::MCStreamer::BeginCOFFSymbolDef(class llvm::MCSymbol const *) c:\ldc\llvm\lib\mc\mcstreamer.cpp:811:0
#5 0x00007ff6dcd96f36 `anonymous namespace'::COFFAsmParser::ParseDirectiveDef c:\ldc\llvm\lib\mc\mcparser\coffasmparser.cpp:428:0
#6 0x00007ff6dcd961d6 llvm::MCAsmParserExtension::HandleDirective<class `anonymous namespace'::COFFAsmParser,&A0x2504f220::COFFAsmParser::ParseDirectiveDef(class llvm::StringRef,class llvm::SMLoc)>(class llvm::MCAsmParserExtension *,class llvm::StringRef,class llvm::SMLoc) c:\ldc\llvm\include\llvm\mc\mcparser\mcasmparserextension.h:39:0
#7 0x00007ff6dcd87713 `anonymous namespace'::AsmParser::parseStatement c:\ldc\llvm\lib\mc\mcparser\asmparser.cpp:1763:0
#8 0x00007ff6dcd73e49 `anonymous namespace'::AsmParser::Run c:\ldc\llvm\lib\mc\mcparser\asmparser.cpp:764:0
#9 0x00007ff6dcbd46a0 llvm::ModuleSymbolTable::CollectAsmSymbols(class llvm::Module const &,class llvm::function_ref<void (class llvm::StringRef,enum llvm::object::BasicSymbolRef::Flags)>) c:\ldc\llvm\lib\object\modulesymboltable.cpp:195:0
#10 0x00007ff6dcbd5240 llvm::ModuleSymbolTable::addModule(class llvm::Module *) c:\ldc\llvm\lib\object\modulesymboltable.cpp:70:0
#11 0x00007ff6dcbd706c `anonymous namespace'::Builder::addModule c:\ldc\llvm\lib\object\irsymtab.cpp:120:0
#12 0x00007ff6dcbd7fc0 `anonymous namespace'::Builder::build c:\ldc\llvm\lib\object\irsymtab.cpp:257:0
#13 0x00007ff6dcbd83ad llvm::irsymtab::build(class llvm::ArrayRef<class llvm::Module *>,class llvm::SmallVector<char,0> &,class llvm::StringTableBuilder &,class llvm::BumpPtrAllocatorImpl<class llvm::MallocAllocator,4096,4096> &) c:\ldc\llvm\lib\object\irsymtab.cpp:280:0
#14 0x00007ff6dca157ee llvm::BitcodeWriter::writeSymtab(void) c:\ldc\llvm\lib\bitcode\writer\bitcodewriter.cpp:3875:0
#15 0x00007ff6dc9fea9b llvm::WriteBitcodeToFile(class llvm::Module const *,class llvm::raw_ostream &,bool,class llvm::ModuleSummaryIndex const *,bool,class std::array<unsigned int,5> *) c:\ldc\llvm\lib\bitcode\writer\bitcodewriter.cpp:3949:0
#16 0x00007ff6dba6017f writeModule(class llvm::Module *,char const *) c:\ldc\ldc\driver\toobj.cpp:381:0

[JohanEngelen]

could this option be generalized to set the linker for any target?
(can be useful for e.g. choosing between ld.bfd and ld.gold, or for cross compiling stuff?)

[kinke]

Good idea, using it as -fuse-ld=... arg for the other targets' gcc command line. So it'll be -linker=<lld-link|gold|bfd|lld> etc.

[kinke]

Implemented.

[PetarKirov]

Looks good to me.