Clang now builds world and kernel, on i386 and amd64

Hi,

As of r212979, you should now be able to build world and kernel on i386
and amd64 with clang, without any additional patches!

To do so, make sure you have updated your installed world to at least
r212904 (which has the most recently imported clang/llvm snapshot), and
put the following in /etc/src.conf:

.if !defined(CC) || ${CC} == "cc"
CC=clang
.endif
.if !defined(CXX) || ${CXX} == "c++"
CXX=clang++
.endif
# Don't die on warnings
NO_WERROR=
WERROR=

Both world and kernel can also be installed, and should run properly,
but please make sure you have a way to revert if anything unexpected
happens. :)  Alternatively, just install into a chroot to try it out
from there.

Some additional information can be found on this wiki page:

http://wiki.freebsd.org/BuildingFreeBSDWithClang

Thanks to all the people that made this possible, especially Roman
Divacky, Ed Schouten, Rui Paulo, and of course the clang/llvm
developers.