Use KSTART_DC in dispatch()
Use KSTART_DC in dispatch()

Profiling shows that get_thread_private_dcontext() takes 1% of the
time on -native_exec DumpRenderTree.  Currently -native_exec ends up
in dispatch far more often than we would if everything were linked
together in the cache.

Review URL: https://codereview.appspot.com/7376057

git-svn-id: ed337ca7cdb08c0ba29d8fe58632bb7686892c2d@1944 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6

rnk/dynamorio,rnk/dynamorio,rnk/dynamorio,rnk/dynamorio,rnk/dynamorio