Update and trim README.newarch.  Leave directions for building externals with an alternate version of dyninst.
Update and trim README.newarch.  Leave directions for building
externals with an alternate version of dyninst.

HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit