x86_to_x64 does not support 32-bit fragments

_From [ya...@google.com](https://code.google.com/u/117083146278203987623/) on August 06, 2012 14:18:06_

Currently, we assume that all fragments are 64-bit for -x86_to_x64. So, for now, X64_CACHE_MODE_DC(dc) is defined as (X64_MODE_DC(dc) || DYNAMO_OPTION(x86_to_x64)), and we always use x64 gencode.

We need to change them if we want to support 32-bit fragments in x86_to_x64.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=862_