add powerpc soft-float support

Some PowerPC CPUs (e.g. Freescale MPC85xx) have a completely different
instruction set for floating point operations (SPE).
Executing regular PowerPC floating point instructions results in
"Illegal instruction" errors.

Make it possible to run these devices in soft-float mode.
diff --git a/configure b/configure
index 89e0d17..9c0762c 100755
--- a/configure
+++ b/configure
@@ -621,6 +621,10 @@
 trycppif __mips_soft_float "$t" && SUBARCH=${SUBARCH}-sf
 fi
 
+if test "$ARCH" = "powerpc" ; then
+trycppif _SOFT_FLOAT "$t" && SUBARCH=${SUBARCH}-sf
+fi
+
 test "$ARCH" = "microblaze" && trycppif __MICROBLAZEEL__ "$t" \
 && SUBARCH=${SUBARCH}el