Do not disable --symbolic-links on Valgrind (or MSAN)

The option --symbolic-links was originally disabled by default under
Purify (and later Valgrind) in 51156c5a
without any explanation.

mysql-test/main/symlink-myisam-11902.test

@@ -56,5 +56,8 @@ flush tables;
 drop table if exists t1;
 exec rm -r $MYSQLTEST_VARDIR/tmp/foo;
 
+# Under Valgrind, this could display a warning ER_FILE_NOT_FOUND
+--disable_warnings
 drop table mysql.t1;
+--enable_warnings
 set debug_sync='RESET';

sql/mysqld.cc

@@ -6729,7 +6729,7 @@ struct my_option my_long_options[]=
      Also disable by default on Windows, due to high overhead for checking .sym 
      files.
    */
-   IF_VALGRIND(0,IF_WIN(0,1)), 0, 0, 0, 0, 0},
+   IF_WIN(0,1), 0, 0, 0, 0, 0},
   {"sysdate-is-now", 0,
    "Non-default option to alias SYSDATE() to NOW() to make it safe-replicable. "
    "Since 5.0, SYSDATE() returns a `dynamic' value different for different "