Start looking for LLVM from version 3.3 then higher version.

Start looking for LLVM from version 3.3 then higher version.

When different LLVM versions are installed, look for 3.5, 3.4 and 3.3 in
order, then try the system default.

As configuring for 3.1 and 3.2 gives an error now, drop these versions from
the search.

v2:
change to use llvm 3.3 as the preferred version.
update the document accordingly.

Signed-off-by: Simon Richter <Simon.Richter@hogyros.de>
Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>