[gn build] Add build files for llvm-ar, llvm-nm, llvm-objdump, llvm-readelf

Also add build files for deps DebugInfo/Symbolize, ToolDrivers/dll-tool.
Also add gn/build/libs/xar (needed by llvm-objdump).

Also delete an incorrect part of the symlink description in //BUILD.gn (it used
to be true before I made the symlink step write a stamp file; now it's no
longer true).

These are all binaries needed by check-lld that need symlinks.

Differential Revision: https://reviews.llvm.org/D55743

llvm-svn: 349486