replace witherspoon-tacoma with rainier in CI

[Andrew Geissler]

The witherspoon-tacoma system was a variant of IBM's witherspoon system that
supported the AST2600. We introduced it into CI primarily to give the community
CI coverage of the new AST2600 chip.

IBM has begun shifting it's focus away from witherspoon-tacoma and on to
rainier. Rainier is also an AST2600 based system, has more features enabled
on it, and is the direction of most new IBM development.

I'd like to propose replacing witherspoon-tacoma in our CI jobs with rainier.

Andrew