refactor cross-platform portions of core/x86/instrument.c

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on May 28, 2014 11:23:48_

This is something that's long been on our clean-up list: move the general
client code out of core/x86/instrument.c into core/, or possibly into a new
subdir of core/ (core/client/, core/tool/, core/instrument/) if we'll have
more than one file. Would annotation support belong there? I don't think
so: annotations aren't strictly tied to tool instrumentation and are more
like the start/stop interface (consider "am I under DR?", DGC annotations,
etc.).

We could also consider putting cross-platform IR code into core/ir/.
This could include quite a bit of core/x86/instr.c (the code that just
operates on operand accessors), and core/instrlist.c.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1450_