Fix LDC arm64 build

[lionello]

This improves upon https://github.com/dlang/dmd/pull/13288 and adds a copy of osmodel.mak into this repo, to break the build dependency between the two repos. This allows one to build the tools with ldc, without the need to clone dmd.

This PR also adds a new make var ARCH which is the llvm/dub compatible architecture (ie. x86_64, x86, or aarch64). This replaces the previous subst hack on line 42, which could never recover arm64 from just the MODEL 64.

Related bugzilla, raising a similar issue: https://issues.dlang.org/show_bug.cgi?id=22078

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Bugzilla references

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + tools#441"

[lionello]

@wilzbach @CyberShadow could you guys have a look at this when you have time?

[CyberShadow]

I thought reusing the include file was a feature. Perhaps it would make sense to use the one from Druntime instead, if the DMD repository is not otherwise useful in these circumstances?

[ljmf00]

I thought reusing the include file was a feature. Perhaps it would make sense to use the one from Druntime instead, if the DMD repository is not otherwise useful in these circumstances?

I think the point here is to break the dependency between the two repositories. The current way could not easily do reproducible builds and it is inefficient to clone a whole repo for a single file. Although I think the Makefile is mostly used internally anyway, so we could just have fallbacks to dmd/druntime/phobos and hang if none of them has it, but I think this is the most reasonable way if we want to stick with old GNU Makefiles.

[MoonlightSentinel]

I think the point here is to break the dependency between the two repositories. The current way could not easily do reproducible builds and it is inefficient to clone a whole repo for a single file.

Then maybe download the single file instead of cloning the entire repo?

curl -L -O https://raw.githubusercontent.com/dlang/dmd/master/src/osmodel.mak

The file can even be pinned to a certain reveision:

curl -L -O  https://raw.githubusercontent.com/dlang/dmd/f87bc623ea3b3a722c50ecb0a6c318add1a35916/src/osmodel.mak

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Copying the file here will just "hide" the dependency because future changes will have to be applied to each copy.

[lionello]

I have submitted a PR for the osmodel.mak changes to dmd, but I still think it's excessive to have a cross-repo dependency for such a trivial file.

Once https://github.com/dlang/dmd/pull/13428 is merged I can update this PR to use curl instead of git clone.

[lionello]

The repos changed quite a lot. Closing.