[TOOLS][POWERPC] Fix build breakage in powerpc64/xc_linux_build.c

[TOOLS][POWERPC] Fix build breakage in powerpc64/xc_linux_build.c
The patch "[SOLARIS] Don't build ptrace code on Solaris." from
xen-unstable now causes siginfo.h to be included which
#defines "si_addr" to "_sifields._sigfault.si_addr".
and breaks the build of  powerpc64/xc_linux_build.c.
This patch renames
 si_addr -> start_info_addr
 si      -> start_info (for consistency)
and
 start_info -> si (in load_devtree beuse of the renaming above)
Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>