Don't define KLD_MODULE when build module with kernel

[howard0su_gmail.com]

Summary

This remove the extra overhead when defining KLD_MODULE. The
performance gains from inline atomic(9) and mutex(9) functions.
However the modules built with kernel is not supposed to be used
with other kernel. In practical, this is unlikely happen.

The behavior of building a module standalone is not changed. This
maintain the compactibility for the out-of-tree modules as before.

[jhb]

I tried a variant of this recently and it actually broke several things as there are some places that use KLD_MODULE for behavior that needs to be different even for modules tied to the kernel. I will perhaps add a new KLD_GENERIC_MODULE and replace some of the KLD_MODULE tests with that instead.