Use a stack allocated buffer for private_instr_encode
Use a stack allocated buffer for private_instr_encode

Profiling shows that this is one of the most frequent callers of
common_heap_alloc.

Review URL: https://codereview.appspot.com/7456044

git-svn-id: ed337ca7cdb08c0ba29d8fe58632bb7686892c2d@1970 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6

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