selftests/rseq: Fix ppc32: wrong rseq_cs 32-bit field pointer on big endian

selftests/rseq: Fix ppc32: wrong rseq_cs 32-bit field pointer on big endian

commit 24d1136a29da5953de5c0cbc6c83eb62a1e0bf14 upstream.

ppc32 incorrectly uses padding as rseq_cs pointer field. Fix this by
using the rseq_cs.arch.ptr field.

Use this field across all architectures.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220124171253.22072-9-mathieu.desnoyers@efficios.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>