Add 'volatile'

The variable keyInt is both accessed from normal task context and
interrupt handler, so it should be a volatile variable.
Without this 'volatile', OpenWatcom's optimizer hangs.
sieve32: Separate 32-bit code segment and 16-bit code segment

OpenWatcom's linker and MSC's linker cannot merge 32-bit segment and
16-bit segment into one segment. Separate them.
Remove __far

There's no need to use __far for csseg32. It is placed in the same
data segment.
more support for separated segments
Merge branch 'koron-more-segments' into topic/watcom
Merge branch 'topic/watcom' of https://github.com/k-takata/486 into k-takata-topic/watcom
Merge branch 'k-takata-topic/watcom'