Don't build full clang toolchain or clang extras in stages 1-3 of buildworld

MFC after: 1 week
Reviewed by: dim (as part of a "larger" diff)
Phabric: D1336
Sponsored by: EMC / Isilon Storage Division


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275909 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Fix accidental MK_DEBUG_FILES=no addition to NXBMAKE in r275909

X-MFC with: r275909
Sponsored by: EMC / Isilon Storage Division


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275910 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f