Add llvm-readobj and binutils symlinks to LLVM_TOOLCHAIN_TOOLS

This patch adds llvm-readobj and the binutils symlink for readelf to
LLVM_TOOLCHAIN_TOOLS.

Tvoid *thread, void *attr,hey are required by some (most?)
autoconf-built libraries, adding these allows me to build newlib with
the toolchain generated this way.

Also opened an issue for that some days ago, see
https://bugs.llvm.org/show_bug.cgi?id=50698

Reviewed By: sbc100

Differential Revision: https://reviews.llvm.org/D104957