.github: cross-compile powerpc64 Linux asset

[ee7]

Continue the recent zig cc work, such that the next configlet release will have two new release assets:

configlet_4.0.0-beta.n_linux_powerpc64.tar.gz
configlet_4.0.0-beta.n_linux_powerpc64.tar.gz.minisig

where the archive contains the executable:

$ file ./configlet
./configlet: ELF 64-bit MSB executable, 64-bit PowerPC or cisco 7500, OpenPOWER ELF V2 ABI, version 1 (SYSV), statically linked, stripped

The powerpc64-linux-musl target will have Tier 1 Zig support.

Note that powerpc64 is big endian.

Refs: #24 Closes: #810

---

To-do:

• [ ] Fix configlet lint
• [ ] Fix configlet --help
• [ ] Fix configlet completion
• [ ] Re-check configlet create|fmt|generate|info|sync|uuid|--version