i#1105: Revert "Use a stack allocated buffer for private_instr_encode"
i#1105: Revert "Use a stack allocated buffer for private_instr_encode"

This reverts r1970.  I wasn't able to quickly come up with a good
solution to encoding instr immediates without truncation assertions.

TBR

Review URL: https://codereview.appspot.com/7513046

git-svn-id: ed337ca7cdb08c0ba29d8fe58632bb7686892c2d@1972 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6

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