I got annoyed by the number of warnings that the CheriBSD build was
emitting. It turns out that we are emitting lots of warnings during
bootstrap because bootstrap tools are built with the default compiler
flags and ignore the warnings flags that are set in bsd.sys.mk.
This commit changes MK_WARNS=no back to -DNO_WERROR. We need to pass
-DNO_WERROR, since the system compiler might have new warnings that we
don't know about yet, and we shouldn't fail the build in that case.