[PATCHv2] arm64/entry: Fix arm64-specific rseq brokenness

[Mathias Stearn]

Did the arm64-specific fix to rseq not get backportted to 7.0? We just
ran our test suite against 7.0.10 which has the other rseq fixes and
everything was fine on x86_64, but arm64 was frequently segfaulting.

I had an AI agent look into this and it reported:
"""
The fix (commits b9eac6a9d93c, 82f572449cfe, 99428157dcf3,
411c1cf43039) first appeared in v7.1-rc1 and will be included in
v7.1.0.

The three commits backported to linux-7.0.y:
- 663121edad54 — "rseq: Revert to historical performance killing behaviour"
- d242126fd21a — "rseq: Implement read only ABI enforcement for
optimized RSEQ V2 mode"
- fb742945d61a — "rseq: Reenable performance optimizations conditionally"

The arm64-specific fix (411c1cf43039 "arm64/entry: Fix arm64-specific
rseq brokenness") was not backported to any stable branch — it will
only appear in v7.1.0.
"""

Is it possible to get 411c1cf43039 backported to 7.0 or was it omitted
intentionally?