support mixed-mode x64-x86 on win10

These x64 tests fail on win10 and need investigation if we want to support them:

49 - code_api|win32.mixedmode (Failed)
50 - code_api|win32.x86_to_x64 (Failed)
51 - code_api|win32.x86_to_x64_ibl_opt (Failed)

Start by reproducing the three failing x64 tests on Windows 10: code_api|win32.mixedmode, code_api|win32.x86_to_x64, and code_api|win32.x86_to_x64_ibl_opt. Investigate why these mixed-mode cases fail and document what support requires; done means the intended cases pass or their support status is clearly established.

Written by the indexing model from the issue text.