tcg-ppc64: Fix compile errors for userspace only builds with gcc 4.6

tcg/ppc64/tcg-target.c has a couple of places where variables are set
unconditionally, but otherwise used only for softmmu builds, not
userspace only builds.  This causes compiler warnings (which are fatal
by default) when compiling for a ppc64 host with gcc 4.6.  This patch
fixes the problem by moving the code which defines and sets the
variables into the CONFIG_SOFTMMU guarded regions.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
