Restartable sequences: Wire up powerpc system call

Wire up the rseq system call on powerpc.

This provides an ABI improving the speed of a user-space getcpu
operation on powerpc by skipping the getcpu system call on the fast
path, as well as improving the speed of user-space operations on per-cpu
data compared to using load-reservation/store-conditional atomics.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>