Building with the standard system headers isn't a perfect match to the
stand environment. Instead, copy over the files we know are safe to
use and constrain what else is used. We use -nostdinc to achieve this.
This also fixes issues with building 32-bit libraries on amd64
sometimes pulling in the wrong cpufunc.h giving an error now that we
stop on errors. It will also enable an easier transition to lua boot.
Sponsored by: Netflix
Default WARNS to 0 still, since there's still some warnings on other
architectures.
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D13301
Move kernel includes and libsa includes together at the top of defs.mk
Move all machine specific defines from Makefile.inc to their friends
in defs.mk.
Add comments and remove now useless junk after the move.
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