selftests/rseq: Validate legacy behavior

selftests/rseq: Validate legacy behavior

The RSEQ legacy mode behavior requires that the ID fields in the rseq
region are unconditionally updated on every context switch and before
signal delivery even if not required by the ABI specification.

To ensure that this behavior is preserved for legacy users in the future,
add a test which validates that with a sleep() and a signal sent to self.

Provide a run script which prevents GLIBC from registering a RSEQ region,
so that the test can register it's own legacy sized region.

Fixes: 566d8015f7ee ("rseq: Avoid CPU/MM CID updates when no event pending")
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://patch.msgid.link/20260428224427.764705536%40kernel.org
Cc: stable@vger.kernel.org