selftests/rseq: Introduce thread pointer getters

selftests/rseq: Introduce thread pointer getters

commit 23a85e9d6f05f64f42bf817d0afa03ea66dc709c upstream.

This is done in preparation for the selftest uplift to become compatible
with glibc-2.35.

glibc-2.35 exposes the rseq per-thread data in the TCB, accessible
at an offset from the thread pointer.

The toolchains do not implement accessing the thread pointer on all
architectures. Provide thread pointer getters for ppc and x86 which
lack (or lacked until recently) toolchain support.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220124171253.22072-7-mathieu.desnoyers@efficios.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>