for mixed-mode (including -x86_to_x64), far ibl must preserve 64-bit regs

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on August 09, 2012 11:00:22_

r8 - r15 are preserved across mode changes, and it's possible (though quite unlikely) that the app is relying on that. (we of course plan to rely on that for clients in mixed-mode.)

for full transparency, 64-bit far ibl targeting 32-bit has to save r8 - r16 , and 32-bit far ibl targeting 64-bit has to restore them (b/c 64-bit
DR in between will clobber them)

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=865_