target-sparc: fix up niagara machine

target-sparc: fix up niagara machine

Remove the Niagara stub implementation from sun4u.c and add a machine,
compatible with Legion simulator from the OpenSPARC T1 project.

The machine uses the firmware supplied with the OpenSPARC T1 project,
http://download.oracle.com/technetwork/systems/opensparc/OpenSPARCT1_Arch.1.5.tar.bz2
in the directory S10image/, and is able to boot the supplied Solaris 10 image.

Note that for compatibility with the naming conventions for SPARC machines
the new machine name is lowercase niagara.

Signed-off-by: Artyom Tarasenko <atar4qemu@gmail.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>