enabling cpu "features" in custom kernel

[Da Rock]

I'm just trying to get my head around kernel building and clang.

If I want to enable certain features available on my cpu which are 
available by clang, how do I enable them? As I understand it using 
ccflags is a no no (ie /etc/make.conf), so how do I go about enabling 
them? I seem to be missing something in my research here...

One thing I did consider was that there might be some "include" or 
"option" somewhere that I missed, but I still can't find it. There must 
be something in the mk files or such, and I'm guessing that its due to 
the ability to build the freebsd with various workarounds for different 
systems - but how would I get around that so I can try testing something 
out?

Cheers