Restartable sequences: wire up x86 32/64 system call

Restartable sequences: wire up x86 32/64 system call

Wire up the rseq system call on x86 32/64.

This provides an ABI improving the speed of a user-space getcpu
operation on x86 by removing the need to perform a function call, "lsl"
instruction, or system call on the fast path, as well as improving the
speed of user-space operations on per-cpu data.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Russell King <linux@arm.linux.org.uk>
CC: Catalin Marinas <catalin.marinas@arm.com>
CC: Will Deacon <will.deacon@arm.com>
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CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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CC: Linus Torvalds <torvalds@linux-foundation.org>
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CC: Boqun Feng <boqun.feng@gmail.com>
CC: linux-api@vger.kernel.org