sim: ppc: move libsim.a creation to top-level

The objects are still compiled in the subdir, but the creation of the
archive itself is in the top-level.  This is a required step before we
can move compilation itself up, and makes it easier to review.

The downside is that each object compile is a recursive make instead of
a single one.  It adds some overhead, so it's not great, but it shouldn't
be a big deal.  This will go away once compilation is hoisted up.

sim/configure.ac

@@ -64,7 +64,6 @@ m4_define([SIM_BUILD_TARGET], [dnl
   AS_VAR_APPEND([SIM_ENABLED_ARCHES], [" $1"])
   m4_if($2, [true], [dnl
     AC_CONFIG_FILES($1/Makefile)
-    subdirs="$subdirs $1"
   ])
   AC_CONFIG_FILES($1/.gdbinit:common/gdbinit.in)
 ])