selftests/rseq: check if libc rseq support is registered

[ Upstream commit d1a997ba ]

When checking for libc rseq support in the library constructor, don't
only depend on the symbols presence, check that the registration was
completed.

This targets a scenario where the libc has rseq support but it is not
wired for the current architecture in 'bits/rseq.h', we want to fallback
to our internal registration mechanism.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/r/20220614154830.1367382-4-mjeanson@efficios.com


Stable-dep-of: 3bcbc209 ("selftests/rseq: Play nice with binaries statically linked against glibc 2.35+")
Signed-off-by: Sasha Levin <sashal@kernel.org>