Add `llvm-readelf` to clang base image.

[mvanotti]

After building clang, a lot of the binaries are removed from the build.

One of them is llvm-readelf, which is be useful to interact with elf files in the images, as it allows for exporting information in JSON format.

[jonathanmetzman]

How big are these? We remove them to make things faster when pulling the images.