Switching to GASNet 1.20.2

Making GASNet 1.20.2 the default for a few nights of testing before
blowing away 1.20.0 to make sure there are no surprises.



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21802 3a8e244f-b0f2-452b-bcba-4c88e055c3ca