Replace gcc-specific __attribute__

[Gregory McGarry <g.mcgarry%ieee.org@localhost>]

I have a patch to replace most uses of gcc-specific __attribute__  
with the BSD sys/cdef.h preprocessor macros.  The most common  
substitution is __attribute__((packed)) with __packed.  These changes  
are mostly mechanical and are a large step towards compiling the  
kernel with PCC.

It would be nice to have these changes before the 5.0 release, but  
I'm not too concerned.  Should they go in?

The patch is here:

http://ftp.netbsd.org/pub/NetBSD/misc/gmcgarry/attribute.patch