Make backend configuration platform-independent

[Geod24]

If we want the backend to support cross compilation, it needs to receive the target platform from the driver.
So the first step is actually in backconfig. In the process of removing static condition, I removed duplicated functions.
Better reviewed commit-by-commit and with whitespace diff off (it makes the diff rather trivial, except for the util_set{32,64} merge).

[dlang-bot]

Thanks for your pull request, @Geod24!

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Testing this PR locally

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dub run digger -- build "master + dmd#12137"

[PetarKirov]

Did you check if dmc still uses this function? If so, it would be worth mentioning in the commit message.

[Geod24]

There's no way to check this, so I wanted to get @WalterBright 's opinion.

[ibuclaw]

DMC is open sourced on the digitalmars github org IIRC, so it can be checked.

[WalterBright]

Please:

1. These sorts of changes make my life difficult as I have to redo them to ensure they work with the other backend's Digital Mars builds. For example, you removed the 16 bit support. Dead code to DMD may not be dead code for DMC. See https://github.com/DigitalMars/Compiler/tree/master/dm/src/dmc
2. This PR is way too large. It can be done incrementally with much easier to see increments. I understand it is done with a series of commits, but those get squashed and lost. It is really not a problem to do a series of PRs if you let us know where it's headed.
3. I was working towards this myself recently with a series (not all in one) PRs already merged. I had to put it on the shelf in order to get some other serious bug reports resolved, and take over things like @SSoulaimane 's abandoned PR. I want to get it done because it will help me in debugging other platforms without needing a setup for those platforms.

[Geod24]

These sorts of changes make my life difficult as I have to redo them to ensure they work with the other backend's Digital Mars builds. For example, you removed the 16 bit support. Dead code to DMD may not be dead code for DMC. See https://github.com/DigitalMars/Compiler/tree/master/dm/src/dmc

Thanks, I've been looking for this for a while. I'll see if there's a way to set it up that does not break DMC all the time.

This PR is way too large. It can be done incrementally with much easier to see increments. I understand it is done with a series of commits, but those get squashed and lost. It is really not a problem to do a series of PRs if you let us know where it's headed.

Commits do not get squashed unless someone squash them, which would be a mistake. But point taken, I'll submit them separately.

I was working towards this myself recently with a series (not all in one) PRs already merged. I had to put it on the shelf in order to get some other serious bug reports resolved, and take over things like @SSoulaimane 's abandoned PR. I want to get it done because it will help me in debugging other platforms without needing a setup for those platforms.

Which is why I actually done this, as I imagined it'd be uncontroversial.

[Geod24]

@WalterBright : I hope DigitalMars/Compiler#12 can be a good way forward.

[thewilsonator]

platform independence has been achieved by #12508 closing. Please reopen if this adds something that PR doesn't fix