[PECOFF] Fixed a bug which caused DataDirectoryChunk to write to a wrong offset in the file buffer

Patch by Ron Ofir.
[PECOFF] Replace magic number with llvm::COFF::DataDirectoryIndex value

Patch by Ron Ofir.
[PECOFF] Don't set IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE if "-fixed" is provided

Based on Ron Ofir's patch.