[builtins] Align muldc3 test with mulsc3

[arichardson]

Summary

This test fails for when targeting i386 (but not x86_64). Copying the
tolerance logic from test_mutlsc3 allows it to pass. Adding this tolerace
does not seem ideal to me, but is the simplest solution to fix the test
failure.

[arichardson]

ping?

compiler-rt/test/builtins/Unit/muldc3_test.c

case zero:
         break;
     case non_zero:
         switch (classify(divisor))
         {
         case zero:
             if (classify(r) != zero)
                 return 1;
             break;
-        case non_zero:
+        case non_zero: {
+
             if (classify(r) != non_zero)
                 return 1;
-            if (r != a * c - b * d + _Complex_I*(a * d + b * c))
+            double _Complex z = a * c - b * d + _Complex_I*(a * d + b * c);
+            // relaxed tolerance to arbitrary (1.e-6) amount.
+            if (cabs((r-z)/r) > 1.e-6)
                 return 1;
             break;
+        }
         case inf:
             if (classify(r) != inf)
                 return 1;
             break;
         case NaN:
             if (classify(r) != NaN)
                 return 1;
             break;