Fix clean targets to remove all generated files

[nirmal-mina]

Summary

Improves make clean behavior in multiple lecture directories.

Changes

• 05/Makefile: remove the generated fsync binary during clean.
• 08/Makefile: use rm -f so clean does not fail when files are missing.
• 09/Makefile: remove generated .s assembly files such as:
  • atomic.s
  • peterson-breaks.s
  • peterson-fence.s

Testing

• Ran make.
• Ran make clean.
• Verified generated binaries and assembly files are removed.
• Verified clean succeeds even when targets are already absent.