I misunderstood the ELF specification for version symbols (verneed):
The `vn_aux` pointer is relative to the main verneed record, not the
start of the section.
This caused many symbols to not be versioned properly in the return
value of `elf.dyn_symbols`. This was discovered in #21454.
Fix it by correcting the offset computation.
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.
