add API routines to spill and restore xmm registers to our spill slots

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on March 10, 2011 21:06:20_

this was PR 315327

we should provide save and restore xmm register routines that use our dcontext spill slots. We don't use them much so there's little risk of collision and they are faster to access then the client could manage using the provided client tls (unless taking a ton of slots via calloc).

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=412_