[tools] Add option to install binutils symlinks

The LLVM tools can be used as a replacement for binutils, in which case
it's convenient to create symlinks with the binutils names. Add support
for these symlinks in the build system. As with any other llvm tool
symlinks, the user can limit the installed symlinks by only adding the
desired ones to `LLVM_TOOLCHAIN_TOOLS`.

Differential Revision: https://reviews.llvm.org/D39530

llvm-svn: 317272