drreg: register spilling mediation framework

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on July 10, 2011 16:52:53_

we plan to build a "drreg" extension that provides register liveness
analysis and stealing. one thing we didn't plan to do but we may want to
is to add a feature to permanently steal a register. when only need access
to a few fields though, directly-addressable TLS should be more performant:
but when have a lot of fields a stolen register could be more efficient.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=511_