Merge bitcoin/bitcoin#21799: guix: Use `gcc-8` across the board

c90f6e51094a1ba4fb2aab35b78f23b6fda645d0 guix: Consistently use gcc-8 for $HOST (Carl Dong)

Pull request description:

  Only non-base commit is the last commit: https://github.com/bitcoin/bitcoin/commit/b5abb07d0d8051f337df2e7981d42a60e0140ee3

  Right now, here's what we use in Gitian:
  - Linux: Focal's [`g++-8-<arch>-linux-gnu`](https://packages.ubuntu.com/focal/g++-8-aarch64-linux-gnu) (`8.4.0-3ubuntu1cross1`)
  - MinGW-w64: Focal's [`g++-mingw-w64`](https://packages.ubuntu.com/focal/g++-mingw-w64) (`9.3.0-7ubuntu1+22~exp1ubuntu4`)

  In Guix right now we use `gcc-9` across the board.

  I think it makes more sense to use `gcc-8` across the board, as it doesn't suffer from the `memcmp` bug, and is what debian buster (stable) does, meaning it will be well tested ([`g++-mingw-w64`](https://packages.debian.org/buster/g++-mingw-w64), [`g++-aarch64-linux-gnu`](https://packages.debian.org/buster/g++-aarch64-linux-gnu)).

  We can accomplish this somewhat easily using Guix as we have tighter control over the toolchain (see: https://github.com/bitcoin/bitcoin/commit/b5abb07d0d8051f337df2e7981d42a60e0140ee3).

  Let me know your thoughts!

ACKs for top commit:
  MarcoFalke:
    Approach ACK c90f6e51094a1ba4fb2aab35b78f23b6fda645d0, haven't reviewed
  laanwj:
    Code review ACK c90f6e51094a1ba4fb2aab35b78f23b6fda645d0
  hebasto:
    ACK c90f6e51094a1ba4fb2aab35b78f23b6fda645d0, I have reviewed the code and it looks OK, I agree it can be merged.

Tree-SHA512: 3e5b9297305232273323aa745ec417ed1be2418ead0e432db7742f5d5f45efe6e4a2ed44328731512cff4bfde80e5f2dc350a131b8b8fb9207a2ef66bce27ed2

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