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. I’ve checked that this works on the cross-compile output for all ELF platforms supported by Bitcoin Core at the moment, as well as PPC64 LE and BE.
