Set _ppc64_native_is_best = True in rpmUtils.arch. (RhBug:1062390)

This is yum default.
Add rpmUtils.arch tests.

Related: RhBug:1062390
Add missing yum arches to rpmUtils.arch; fix getBaseArch().

Related: RhBug:1062390
Make UnicodeStream Python 2 compatible (RhBug:1063022)

Partially reverts patch aa74c30 which assumed that stdout is unicode-based while it is str-based.
api: add Repo.pkgdir.