powerpc: Wire up restartable sequences 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 <[email protected]>
Signed-off-by: Mathieu Desnoyers <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Joel Fernandes <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Dave Watson <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: "H . Peter Anvin" <[email protected]>
Cc: Chris Lameter <[email protected]>
Cc: Russell King <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Andrew Hunter <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: "Paul E . McKenney" <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: Josh Triplett <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Ben Maurer <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Andy Lutomirski <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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