ppc64: Don't try to build sPAPR RTAS on Darwin

The Darwin assembler fails to build it.

Cc: David Gibson <[email protected]>
Cc: Alexander Graf <[email protected]>
Signed-off-by: Andreas Färber <[email protected]>
Signed-off-by: Alexander Graf <[email protected]>
diff --git a/configure b/configure
index 6f75e2e..fbf5d5f 100755
--- a/configure
+++ b/configure
@@ -2540,7 +2540,7 @@
         "$softmmu" = yes ; then
   roms="optionrom"
 fi
-if test "$cpu" = "ppc64" ; then
+if test "$cpu" = "ppc64" -a "$targetos" != "Darwin" ; then
   roms="$roms spapr-rtas"
 fi