feat: add libc __exit_funcs utility class

[hannahfluch]

closes #2633

Pwntools Pull Request

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WIP!

Adding a utility class for glibc's __exit_funcs. This can be helpful when promoting an arbitrary write and read to RCE.

See this resource for more info. A lot of exploits just use the CxaFunc and ExitHandler class shown here, so it would make sense to have the functionality in pwntools as well.

TODO

• documentation
• compliance with older glibc versions
• exit handler class

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