luajit: fix on Catalina

[clason]

backport lj_alloc.c from 2.1 branch to 2.0.5, required to make luajit stable work on Catalina

see #46823
and LuaJIT/LuaJIT#521

cc @fxcoudert

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• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
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[fxcoudert]

Thanks @clason
Can you confirm that this build passes locally and fixes your issues?

[clason]

@fxcoudert Yes, I just double-checked. brew install luajit -s compiles and works on Catalina, which in turn fixes neovim (bottled and built).

[clason]

Oh, and do I have to do something special to get the Catalina bottle rebuilt from this formula? The bottle block is autogenerated on merge, right?

[fxcoudert]

Bottles will be automatically regenerated.