preserve ymm registers

From [email protected] on April 13, 2011 11:20:56

issue #396 (closed) is adding AVX + FMA IR support

this issue covers adding ymm register preservation. it's not clear how to do it w/o breaking backward compatibility in the dr_mcontext_t struct.

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=433