labs/lab-12: Add x86_64 version

[teodutu]

Translate the tasks, reading material and guides of this lab to the x86_64 architecture:

• The reading section is quite short and made for 64-bit binaries already, so that can be left unchanged.
• Then change all 32-bit tasks to 64. Make sure the solutions still link properly and update the writeups to 64 bits.
  • You can use gcc -v to see how it invokes the linker and use the same command for the tasks that call ld directly.
  • Don't make any special mentions about 64-bit binaries for the ROP challenge.
• Verify that the checker infrastructure still works and fix it if it doesn't.

Make your PR target the upstream x86_64 branch.

Read the contributor guidelines before making contributions [1].
Add Fixes #88 at the end of your commit message to automatically link your PR with this issue.

[1] https://github.com/cs-pub-ro/hardware-software-interface/blob/main/CONTRIBUTING.md