In emulation of execve DynamoRIO does not check that file is executable

```
$ cat > prog.c <<'EOF'
#include
#include

int main(int argc, char *argv[])
{
printf("Running %s\n", argv[0]);
if (argv[1] != NULL)
if (execv(argv[1], argv + 1))
perror("execv");
return 0;
}
EOF
$ gcc -Wall -O2 prog.c -o prog1
$ cp prog1 prog2
$ chmod -x prog2
$ ./prog1 prog2
Running ./prog1
execv: Permission denied
$ bin64/drrun -- ./prog1 prog2
Running ./prog1
Running prog2
$
```