[PATCH] arm64/entry: Fix arm64-specific rseq brokenness (was: Re: [REGRESSION] rseq: refactoring in v6.19 broke everyone on arm64) and tcmalloc everywhere

[Mark Rutland]

Patch for the arm64-specific issue below. This doesn't fix the generic
cpu_id_start issue, but it brings arm64 into line with everyone else,
and it's the shape we'll need going forwards for other stuff anyway.

I've given it light testing with Mathias's reproducer and the
kselftests, which all pass.

I've also pushed it to my arm64/rseq branch:

  https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/rseq

Mark.

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