Move current ruby22 status into arch profiles, bug 615818

Move current ruby22 status into arch profiles, bug 615818

Mask ruby_targets_ruby22 in the use.stable.mask of each arch and set
RUBY_TARGETS to ruby21 in the make.defaults for each arch.

This makes it possible to update the defaults in the main profile for
a stable ruby:2.2 slot and consequently to mark ruby:2.2 stable on
each arch in turn making sure that users of that arch will not end up
with conflicting settings (e.g. stable ruby:2.2 but
RUBY_TARGETS="ruby21").

Each arch is expected to remove these temporary entries as they mark
ruby:2.2 stable, as per the instructions in bug 615818.