arm64: entry.S convert el0_sync

el0_sync also unmasks exceptions on a case-by-case basis, debug exceptions
are unmasked, unless this was a debug exception. Irqs are unmasked for
some exception types but not for others.

el0_dbg should run with everything masked, for the other cases we can
unmask everything. This changes the behaviour of fpsimd_{acc,exc} and
el0_inv, which previously ran with irqs masked.

All of these exception types call ct_user_exit after unmasking interrupts.
Move this into the switch statement. el0_dbg needs to do this itself once
it has finished its work, el0_svc needs to pass a flag to restore the
syscall args.

This patch removed the last user of enable_dbg_and_irq, remove it.

Signed-off-by: James Morse <james.morse@arm.com>