arm64: rseq: Implement backend rseq calls and select HAVE_RSEQ

Implement calls to rseq_signal_deliver, rseq_handle_notify_resume
and rseq_syscall so that we can select HAVE_RSEQ on arm64.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/entry.S

@@ -904,6 +904,7 @@ ENDPROC(el0_error)
 ret_fast_syscall:
 	disable_daif
 	str	x0, [sp, #S_X0]			// returned x0
+#ifndef CONFIG_DEBUG_RSEQ
 	ldr	x1, [tsk, #TSK_TI_FLAGS]	// re-check for syscall tracing
 	and	x2, x1, #_TIF_SYSCALL_WORK
 	cbnz	x2, ret_fast_syscall_trace
@@ -911,6 +912,7 @@ ret_fast_syscall:
 	cbnz	x2, work_pending
 	enable_step_tsk x1, x2
 	kernel_exit 0
+#endif
 ret_fast_syscall_trace:
 	enable_daif
 	b	__sys_trace_return_skipped	// we already saved x0