Restrict usage of m68k assembler versions of {,u}divsi3 and {,u}divsi3 to
kernel and bootloader for 68010.

They requires a special calling convention to udivsi3, and cannot to be
mixed up in normal routines provided by libgcc or compiler_rt. Although,
there's no problem for using in a controlled situation, i.e., kernel and
standalone programs.

Note that this does not affect m68k ports other than sun2, since codes
generated by gcc do not call these routines.

Assembler files are moved from common/lib/libc/arch/m68k/gen to
sys/lib/libkern/arch/m68k in order not to be compiled in libc.

Revert hack introduced to lib/libc/compiler_rt/Makefile.inc rev 1.37:
http://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libc/compiler_rt/Makefile.inc#rev1.37

Proposed on port-sun2@ with no response...
(Again, this does not affect m68k ports other than sun2.)
http://mail-index.netbsd.org/port-sun2/2020/03/10/msg000102.html

(rin)