Update makellvm to work with the brave new world of separate obj-root trees.

With these changes, you can still run makellvm directly from your source tree
as follows:
  % makellvm [toolname]	## looks for obj-root in default places (see below)
or
  % makellvm -obj obj-root [toolname]	## specify obj-root explicitly.

With the first from, `makellvm' checks the following places in order
and uses the first one that contains the file `Makefile.config':
(1) Your src-root directory
(2) /localhome/$USER/llvm
Other choices can be added to this list.

llvm-svn: 8509