m68k structure packing

[Jim Wilson]

Without this patch, gcc will always generate working code, but will not
pack structures as expected if STRUCTURE_SIZE_BOUNDARY != 8.

With this patch, gcc will pack structures regardless of what
STRUCTURE_SIZE_BOUNDARY is set to, but will now in some cases silently
generate code that fails at run time.

This does not seem like much of an improvement to me.  I think the former
problem is easier to deal with than the latter problem.

Jim