contrib: Parse ELF directly for symbol and security checks

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.

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