New Makefile Features:

* "dist" target now builds tar.gz, tar.bz2, and zip files suitable for
  distribution. "dist" can only be run from $(BUILD_OBJ_ROOT) and implies
  a "check".

* made the preconditions not do a recursive make and ensured that they are
  executed sequentially.

* made the messages output by the makefile be prefixed with "llvm" and the
  make level (e.g. llvm[1]: ) in the same way that make does so that the
  messages are uniform and more readable.

* Fixed the tags target so that tags depends on TAGS which contains the
  rules to build a file named TAGS

* Implemented the EXTRA_DIST feature in a few directories to make sure it
  works.

llvm-svn: 17210