make configure check for unsupported (SPE) powerpc hard-float models

the SPE ABI may be compatible with soft-float, but actually making it
work requires some additional work, so for now it's best to make sure
broken builds don't happen.

configure

@@ -622,6 +622,8 @@ trycppif __mips_soft_float "$t" && SUBARCH=${SUBARCH}-sf
 fi
 
 if test "$ARCH" = "powerpc" ; then
+trycppif "__NO_FPRS__ && !_SOFT_FLOAT" "$t" && fail \
+  "$0: error: compiler's floating point configuration is unsupported"
 trycppif _SOFT_FLOAT "$t" && SUBARCH=${SUBARCH}-sf
 fi