shared code for thread_private code cache

_From [zhao...@google.com](https://code.google.com/u/106321947286816917100/) on April 23, 2013 16:12:39_

Using thread private code cache on windows x86, we do not have shared code.
It is reasonable to have a shared code for both thread-private and shared-shared code cache for for case like out-of-line clean call save and restore.

It looks like shared gencode is created on linux or on x64 already.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1149_