parisc: Provide an extru_safe() macro to extract unsigned bits

The extru instruction leaves the most significant 32 bits of the
target register in an undefined state on PA 2.0 systems.
Provide a macro to safely use extru on 32- and 64-bit machines.

Suggested-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>