mips: display a warning message when using an unknown CPU

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

configure

@@ -4238,6 +4238,8 @@ elif enabled mips; then
             disable msa
         ;;
         *)
+            # Unknown CPU. Disable everything.
+            echo "Warning: unknown CPU. Disabling all MIPS optimizations."
             disable mipsfpu
             disable mips32r2
             disable mips32r6