Fix LLInt vector offsets on Windows 64

Derived classes on Windows x64 are 64bit aligned placing the m_size
property of Vector slightly differently than on other platforms.

Change-Id: I186de5ea200abfbdd3bdf7502f3f427cb6890f3c
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>