Fix rseq fast path on glibc < 2.35: read the shared __rseq_abi area

The own-area approach did not deliver the fast path on glibc 2.31 (YT's current
runtime). There tcmalloc registers the conventional `__rseq_abi` area for every
thread; our attempt to register a separate area was rejected by the kernel with
EINVAL (a thread may have only one rseq area), so `cpu_id` stayed -1 and every
`GetCurrentCpuId()` fell back to `sched_getcpu()` (~17-20 ns, slower than the
rdtscp it replaced).

Read the shared `__rseq_abi` symbol instead -- the area tcmalloc, librseq and
pre-2.35 glibc all register. Our definition is weak, so it coalesces with theirs
when present (the common case -- tcmalloc owns it) and stands alone otherwise
(e.g. musl), with us registering it. We register with the conventional signature
`0x53053053` and size 32, so re-registering an already-registered area returns
EBUSY (treated as success) rather than EINVAL -- coexisting cleanly with tcmalloc.

glibc >= 2.35 still takes the `__rseq_offset` path unchanged.

Measured on sas2-2769 (glibc 2.31 + tcmalloc): `GetCurrentCpuId()` 20.0 ns -> 0.60 ns,
verified via strace that our registration now returns EBUSY against tcmalloc's
`__rseq_abi` (was EINVAL against a separate area).
commit_hash:509809deeb5f7c671817fcd9ebcc8499eabf096e

0 files changed, 0 insertions, 0 deletions