AT_SECURE is set in the auxiliary vector when we execute setuid/setgid
programs.
In those cases, we do not want to read environment variables that
influence the logic of the dynamic loader, as they can be controlled
by the user.
If set, the dynamic loader will perform a software breakpoint after
loading all libraries, and just before jumping to the main entry point.
This allows a debugger to inspect the loaded libraries before the
program starts executing.
A strong symbol anywhere in an executable must override any
weak symbol used in any library. This means that the weak symbol
must be overridden by a strong symbol even if the strong symbol
is in a dependent library. This means we need to perform relocations
twice, and resolve weak symbols globally before attempting to resolve
them locally. Consequentially we need to defer performing any
initialisations until after we have performed the second round of
relocations.
Loader.so now just performs the initial self relocations and static
LibC initialisation before handing over to ELF::DynamicLinker::linker_main
to handle the rest of the process.
As a trade-off, ELF::DynamicLinker needs to be explicitly excluded from
Lagom unless we really want to try writing a cross platform dynamic loader
