Complete DOS support for tools

[OPNA2608]

loadtest finally works on 16-bit DOS now! But we still have abit to go:

• p86extract runs out of allocation memory ~~That allocation name & amount look suspicious as well. Identical in loadtest's attempt at the same large allocation.~~
• Prolly a smarter (read: more automatic) way of handling P86_ImportFile(Slim)
  • CMake option() that sets a define & preprocessor magic for merging P86_ImportFileSlim into P86_ImportFile, or making the latter into a wrapper around the preferred implementation & keeping both ways as separate functions.

[OPNA2608]

Allocation name & amount fixed by 11b0cecbf9edc2931e0e1868afade5afb164727d.

[OPNA2608]

Checklist:

• [x] loadtest
• [ ] p86extract Doesn't crash but reads garbage data in USE_LOWMEM mode.
• [ ] p86create Untested