Removed '@' from delimiters, added it as offset designator.

Moved '@' from delimiters to offset designators for the KMP_PLACE_THREADS
environment variable. Only one of: postfix "o" or prefix @, should be used
in the value of KMP_PLACE_THREADS. For example, '2s@2,4c@2,1t'. This is also
the format of KMP_SETTINGS=1 output now (removed "o" from there).
e.g., 2s,2o,4c,2o,1t.

Differential Revision: http://reviews.llvm.org/D13701

llvm-svn: 250846