linux-user: define correct UTS machine name for hppa

the correct UTS machine name (as expected by systemd) is "parisc",
not "hppa".

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20170126080449.28255-4-laurent@vivier.eu>
Signed-off-by: Richard Henderson <rth@twiddle.net>