arm64: factor work_pending state machine to C

Currently ret_fast_syscall, work_pending, and ret_to_user form an ad-hoc
state machine that can be difficult to reason about due to duplicated
code and a large number of branch targets.

This patch factors the common logic out into the existing
do_notify_resume function, converting the code to C in the process,
making the code more legible.

This patch tries to closely mirror the existing behaviour while using
the usual C control flow primitives. As local_irq_{disable,enable} may
be instrumented, we balance exception entry (where we will almost most
likely enable IRQs) with a call to trace_hardirqs_on just before the
return to userspace.

Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>