Instead of the ever-messier text parsing of the output of the readelf tool (which is clearly meant for human consumption not to be machine parseable), parse the ELF binaries directly. Add a small dependency-less ELF parser specific to the checks. This is slightly more secure, too, because it removes potential ambiguity due to misparsing and changes in the output format of `elfread`. It also allows for stricter and more specific ELF format checks in the future. This removes the build-time dependency for `readelf`. It passes the test-security-check for me locally, though I haven't checked on all platforms.