mips: add i6400 core optimizations

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

configure

@@ -4162,9 +4162,10 @@ elif enabled mips; then
             disable mipsdsp
             disable mipsdspr2
             disable mipsfpu
-
-            check_cflags "-mtune=i6400 -mabi=64"
-            check_ldflags "-mabi=64"
+            check_cflags "-mtune=i6400 -mabi=64" &&
+            check_cflags "-mfp64 -msched-weight -mload-store-pairs -funroll-loops" &&
+            check_ldflags "-mabi=64" &&
+            add_asflags "-mfp64"
         ;;
         loongson*)
             disable mipsfpu