Before of this change, actually setting the m_access to contain the
HasBeen{Readeable,Writable,Executable} bits was done by the method of
Region set_access_bit which added ORing with (access << 4) when enabling
a certain access bit to achieve this.
Now this is changed and when calling set_{readeable,writable,executable}
methods, they will set an appropriate SetOnce flag that could be checked
later.
5194ab59b5