parisc: Add support for CONFIG_SYSCTL_ARCH_UNALIGN_NO_WARN

Allow users to disable kernel warnings for unaligned memory
accesses from kernel via the /proc/sys/kernel/ignore-unaligned-usertrap
procfs entry.
That way users can disable those warnings in case they happen too
often.

Signed-off-by: Helge Deller <deller@gmx.de>