ppc64le: address review comments from mkannwischer

[bhess]

Addresses the latest review comments from @mkannwischer:
pq-code-package#1648 (review)

• remove the liboqs integration from this PR; it can come in a follow-up once validated
• fix the indentation issue
• add the POWER8_VECTOR guard
• update the copyright header as suggested
• add toolchain_ppc64le to cross-autogen
• extend CI with separate POWER7 and POWER8 coverage
• re-run autogen in the nix cross-ppc64le environment

@dannytsen, one remaining item is rebasing onto main, as requested here:
pq-code-package#1648 (review)

I do not currently have permission to rebase or merge main into your branch directly. If you want me to take care of that, please grant me write access to the branch. Otherwise, please rebase it when convenient.

[dannytsen]

@bhess You have the write access now.

[bhess]

@bhess You have the write access now.

Thanks, I'll merge main to the branch then and push that directly to new_main.

[dannytsen]

@bhess You have the write access now.

Thanks, I'll merge main to the branch then and push that directly to new_main.

Thanks.