Update the cross-compiling doc at the top.
Remove sys/msdos/Makefile1.cross, sys/msdos/Makefile2.cross, and
sys/msdos/msdos-cross-compile.sh as they are no longer required.
Remove occurrences of CROSSCOMPILE_HOST as the host-side of a
cross-compile can be determined from:
defined(CROSSCOMPILE) && !defined(CROSSCOMPILE_TARGET)
without the additional macro.
945d10cfbc