Use Math.DivRem in another place in mscorlib

Went through all uses of % looking for places DivRem could be used.  This looks like it's the only one of note.

Author: stephentoub

src/mscorlib/src/System/Globalization/IdnMapping.cs

@@ -710,8 +710,10 @@ static String punycode_encode(String unicode)
                                             k >= bias + tmax ? tmax : k - bias;
                                     if (q < t) break;
                                     Contract.Assert(punycodeBase != t, "[IdnMapping.punycode_encode]Expected punycodeBase (36) to be != t");
-                                    output.Append(encode_digit(t + (q - t) % (punycodeBase - t)));
-                                    q = (q - t) / (punycodeBase - t);
+
+                                    int mod;
+                                    q = Math.DivRem(q - t, punycodeBase - t, out mod);
+                                    output.Append(encode_digit(t + mod));
                                 }
 
                                 output.Append(encode_digit(q));