Rico noticed that V8 ARM builder also fails on HeapEntry size
assertion. As MSVC-specific way of fixing the problem causes
aliasing problems on G++, I re-implemented conversion using
unions. And #ifdefs are gone!

TBR=sgjesse@chromium.org

Review URL: http://codereview.chromium.org/5328001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5872 ce2b1a6d-e550-0410-aec6-3dcde31c8c00