CI: Set up PowerPC 64 bit big endian

Motivation

GHC has several big endian bugs.

Proposal

Set up a PowerPC gitlab-runner on @trommler 's Power Mac. Additionally set up a Power8 and/or POWER9 instance (this is already running in @trommler 's fork.

Details to come.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information