time for MKDTRACE=yes for some ports?

[Jeff Rizzo]

I propose to enable MKDTRACE by default for the following ports:
amd64
i386
(some subset of?) arm

It's made a lot of progress, recently, and will be easier to work on 
going forward with it on by default in places where it works.  I'm not 
currently proposing to enable KDTRACE_HOOKS in GENERIC, but we might 
want to start thinking about that.

Thoughts?  I'd like to do this next week sometime.

+j