[ELF] Add llvm-readelf to the lit tool substitutions

Summary:
Some of the tests invoke llvm-readelf. This currently appears to work
probably because the LLVM binary directory is included in $PATH. However,
this is quite fragile so let's just make lit expand the full path.

Reviewers: ruiu, espindola

Reviewed By: ruiu

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D43827

llvm-svn: 326257