Ensure mjsunit tests use dashes not underscores in flags directives.

test/mjsunit/regress/regress-regexp-codeflush.js

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-// Flags: --gc_global
+// Flags: --gc-global
 
 // Regression test for regexp that has multiple matches and which
 // internally calls RegExpImpl::IrregexpExecOnce more than once without
 // ensuring that the regexp is compiled.
 // This can create a crash if the code was exchanged with the sweep
 // generation (for code flushing support) in GC durring the matching.
 
 var re = new RegExp('(s)', "g");
 
 function foo() {
   return "42";
 }
 
 // Run enough times to get a number of GC's (all mark sweep because of the
 // --gc_global) flag.
 for ( var i = 0; i < 10; i++) {
   // Make a long string with plenty of matches for re.
   var x = "s foo s bar s foo s bar s";
   x = x + x;
   x = x + x;
   x = x + x;
   x = x + x;
   x = x + x;
   x = x + x;
   x = x + x;
   x.replace(re, foo);
 }