for wow64 apps, use 64-bit dbghelp to avoid 32-bit isolation problems

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on November 15, 2012 10:58:42_

we've hit a lot of private library isolation issues as we scale up Dr. Memory:
- issue #963 + issue #964 + issue #967 + issue #977 + issue #915 presumably, for wow64, messing w/ the x64 PEB and TEB and system library
state is much less likely to cause a problem, as opposed to messing with
the app system state while interrupted the app at arbitrary times.
so the idea is to forget loading duplicate 32-bit copies and instead use
the native 64-bit versions.

one approach is to use a 64-bit drmemorylib operating on 32-bit chrome
code: xref the mixedmode plans for that and complications (replace_\* need
to be 32-bit, need to finalize DR mixed-mode interface ( issue #751 ) and handle 64-bit app code or else change our plans there and support a 64-bit tool that only looks at the 32-bit code, etc.)

a simple alternative is to have an interface library that forwards 32-bit
dbghelp API to 64-bit dbghelp. put it inside drsyms. special build of
drsyms that doesn't import from dbghelp.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=979_