arm64: unmask all exceptions on CPU startup

Remove the local_{async,fiq}_{en,dis}able macros as they don't respect
our newly defined order, and only have one user:
CPU 0 unmasks SError during early boot once it can print an error message.

Debug exceptions are already unmasked by __cpu_setup(), which has also
configured MDSCR_EL1 to disable MDE and KDE.

Change the local_async_enable() call to unmask everything except irqs.
If we can print an error message about SError, we can do the same for FIQ.
The same goes for secondary CPUs only here we are ready to receive
interrupts. Just unmask everything.

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