Ensure mjsunit tests use dashes not underscores in flags directives.

test/mjsunit/math-floor-negative.js

 // Copyright 2012 the V8 project authors. All rights reserved.
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-// Flags: --noenable_sse4_1 --allow-natives-syntax
+// Flags: --noenable-sse4-1 --allow-natives-syntax
 
 function test1() {
   // Trigger overflow when converting/truncating double to integer.
   // Divide by 10 to avoid overflow when smi-tagging at the end.
   return Math.floor(-100000000000.5) / 10;
 }
 
 function test2() {
   // Trigger no overflow.
   return Math.floor(-100.2);
@@ skipping 11 matching lines @@
 test2();
 test2();
 %OptimizeFunctionOnNextCall(test2);
 test3();
 test3();
 %OptimizeFunctionOnNextCall(test3);
 
 assertEquals(-10000000000.1, test1());
 assertEquals(-101, test2());
 assertEquals(-214748364.9, test3());