[LoongArch64] Set up loongarch.loongarch64 architecture

[haowuge]

This is the initial set up for LoongArch64.
The current plan is for this arch to remain experimental for FreeBSD 15.
This is our amateur learning project, and we will gradually improve it over time.

Regarding the modifications related to the contrib/llvm-project section,
my colleague will submit a pull request to the llvm-project later.
A simple GitHub Actions build test: https://github.com/haowuge/freebsd-src/actions/runs/12178365925/job/33968197600

If you have better suggestions for the code, please let me know,
and we are happy to improve it until compliant code is provided.

[bsdimp]

So I've taken a quick look. Generally I like it.
However, it's done all as one big commit. There's at least two commits here. (1) all the Makefile related stuff and (2) Adding Loongson target support to clang/lld/etc.
I'll look at it in more detail to see if there's improvements I can suggest. Nothing stood out in my first, admittedly hasity, pass through the patch which is a good sign.

[brooksdavis]

Somewhat echoing @bsdimp, I'd like to see a PR that adds general support and works with CROSS_TOOLCHAIN before we add build time to the default toolchain build. IMO in-tree compiler support ranks close to last in bringup priorities. Once we're a bit further along I'd be happy to add LoongArch to FREEBSD_BACKENDS in devel/llvm19.

[haowuge]

Somewhat echoing @bsdimp, I'd like to see a PR that adds general support and works with CROSS_TOOLCHAIN before we add build time to the default toolchain build. IMO in-tree compiler support ranks close to last in bringup priorities. Once we're a bit further along I'd be happy to add LoongArch to FREEBSD_BACKENDS in devel/llvm19.

We originally hoped to merge into the basic system to reduce the pressure of tracking the main branch.

I will try to create LoongArch ports for LLVM 19, and this pull request will temporarily be turned into a draft,
thank you both.

[brooksdavis]

I will try to create LoongArch ports for LLVM 19, and this pull request will temporarily be turned into a draft, thank you both.

You shouldn't need to do anything to make the existing devel/llvm19 package work as it includes all non-experimental backends. If you wanted llvm19-lite to work I'd need to add LoongArch to the list of FreeBSD arches, but that's mostly a long term CI thing.

[haowuge]

I will try to create LoongArch ports for LLVM 19, and this pull request will temporarily be turned into a draft, thank you both.

You shouldn't need to do anything to make the existing devel/llvm19 package work as it includes all non-experimental backends. If you wanted llvm19-lite to work I'd need to add LoongArch to the list of FreeBSD arches, but that's mostly a long term CI thing.

We have several patches for loongarch FreeBSD, so the devel/llvm19 package needs to be patched.
we also made a simple devel/llvm19 ports patch:
0001-add-clang-LoongArch-Add-FreeBSD-targets-119191-patch.patch

We attempted to submit the patches to the LLVM project llvm/llvm-project#119191 , and the upstream suggested that FreeBSD should have more specialized personnel review it. Could you please help check if the patches meet the requirements?

Thank you.