Fix most reproducibility issues

This MR fixes most of the outstanding reproducibility issues, but not all.

I built the full package using reprotest, disabling the fileordering variation (which I couldn't get to work in a container) and the time variation (which seemed to cause an infinite loop of Makefile updating).

diffoscope needed more than the available 16 GB RAM to compare all packages, but I have been able to compare binary packages one-by-one and only look at the "cloud-amd64" flavour in depth.

The changes are:

• Revert "perf tools: Use $KBUILD_BUILD_TIMESTAMP as man page date". The actual fixes for man page reproducibility are upstream since 6.5-rc1, so this just removes our old broken patch for it.
• d/rules.real: Fix missing -ffile-prefix-map options for user-space
• linux-perf: Remove remaining source filename from executable
• linux-perf: Work around gcc bug with -ffile-prefix-map and assembly sources
• Documentation: Use relative source filenames in ABI documentation
• kbuild, bpf: Fix BTF reproducibility
• d/rules.real: Add -fdebug-prefix-map option for assembly sources in kernel

The remaining problems are:

• linux-doc-6.11:
  • In html/translations/zh_TW/process/license-rules.html, the "current" class is applied to some different elements.
  • The sidebar ToC sometimes changes.
• linux-image-6.11+unreleased-cloud-amd64-unsigned:
  • The built-in certificate changes.
  • Module signatures change.
• linux-image-6.11+unreleased-cloud-amd64-dbg:
  • diffoscope OOMs when trying to compare this but a simple diff of the contents shows that only vmlinux differs.

This is currently based on top of !1213 (merged), which is needed to make objtool reproducible.