DOS is not the same thing as the underlying CPU, after all. A separate
file not only indicates to future port authors which parts of the code
are x86-specific, but it also speeds up build times…
… in theory, because removing 677 lines from 49 files each doesn't seem
to speed up the build as much as I had hoped? But apparently my whole
system mysteriously got faster in the meantime, and I was getting 22-23
seconds for the entire repo even before this commit. Good enough.
Part of P0134, funded by [Anonymous].
Might seem inconsistent, given that the function initializes pointers
that are declared in planar.h… but it's always called from other game
initialization functions that don't require pc98.h or planar.h.
Part of P0133, funded by [Anonymous].