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
Fix accidental MK_DEBUG_FILES=no addition to NXBMAKE in r275909

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