Warn users on 32-bit CPUs and big-endian CPUs

[ChrisJefferson]

This PR adds a message which is displayed whenever GAP is started on a 32-bit CPU, or a big-endian CPU, pointing out that GAP don't support this CPU, and we aren't doing full testing.

There are I feel two questions here:

1. Is adding such a message reasonable? I think it is, because in general people want to trust the answers they get from GAP.

2. Is this message right? Here's what the output looks like:

 ┌───────┐   GAP 4.13dev-42-g7cf6fb7 built on 2022-09-29 20:35:04+0100
 │  GAP  │   https://www.gap-system.org
 └───────┘   Architecture: x86_64-pc-linux-gnu-default64-kv8

 **** Warning, this copy of GAP was built for: 32-bit CPUs
 **** This configuration is not officially supported or tested

 Configuration:  gmp 6.2.1, GASMAN, readline
 Loading the library and packages ...

[fingolfin]

I am fine with this in principle, but we should not do this silently. Instead I'd suggest to bring this PR up on the GAP mailing list, so that people have a chance to react and comment. Do you want to do that, @ChrisJefferson ?

This also ties in with #5026 ...

[fingolfin]

If we do this, we might as well also drop the (soon broken) 32bit package tests from CI; see also discussion in PR #5026.