Setup the sync with LuaJIT upstream

[igormunkin]

There is no regular sync since LuaJIT/LuaJIT@8271c64 (i.e. v2.1.0-beta3). In scope of this issue we need to:

• develop and publish the backporting scenario (Continuous backporting of new commits from Luajit - set up procedure #5534)
• backport everything from upstream since the mentioned revision to 2.1 head
  • downstream all aarch64-related patches from vanilla LuaJIT repo (Downstream all aarch64-related patches from vanilla LuaJIT repo #5629)
  • Backporting LuaJIT upstream commits part 1  #6548
  • Backporting LuaJIT upstream commits part 2 #7230
  • Backporting LuaJIT upstream commits part 3 #8069
  • Backporting LuaJIT upstream commits part 4 #8516
  • Backporting LuaJIT upstream commits part 5 #8825
  • Backporting LuaJIT upstream commits part 6 #9145
  • Backporting LuaJIT upstream commits part 7 #9595
  • Backporting LuaJIT upstream commits part 8 #9924
• describe and setup the continuous backporting procedure for further sync

Current status is available here

[igormunkin]

Moving the valuable inputs by @Totktonada from #5534:

I would like to know where I can look results of upstream patches review and the overall status of backporting (how long non-analyzed queue is, how many patches that looks desired, but have no tests yet, and so on). Say, if you considered a patch as unrelevant to our current goals, I want to know a reason. At least I don't want to be in the situation, when I need to ask the team each time I wonder about a particular patch — whether someone analyzed it? or it was missed unintentionally? or it is in the queue to analyze?

I have another example of status tracking: we did track the remaining tasks and overall status using GitHub wiki. The situation is quite similar: the predefined large scope of work and wish to understand where we're on the way.

I believe all questions above should be resolved within the third bullet of this issue.