Add initial support for LoongArch64

[wszqkzqk]

LoongArch is a new RISC ISA developed by loongson. There are already a lot of community support and testing about it.

This PR add loong64 support to the pure C implementation. I've tested this PR on Arch Linux for Loong64 with gcc 14.2.1 (with additional patch to fix Q suffix for fp128 literals) and clang 19.1.7. Here is the log:

• sleef-build-with-gcc-loong64.log
• sleef-build-with-clang-loong64.log

I also tested it by rebasing and applying it to pytorch 2.6.0. I've tested it with some python demos and it works fine.

[shibatch]

If we decide to officially support LoongArch, then we need to keep maintaining it.
To do so, we also need to maintain a CI environment.
So far, I have taken on the work for free, but I cannot do it any more.
We need you to bear the labor and expense for this.

[wszqkzqk]

If we decide to officially support LoongArch, then we need to keep maintaining it. To do so, we also need to maintain a CI environment. So far, I have taken on the work for free, but I cannot do it any more. We need you to bear the labor and expense for this.

I cannot afford the CI since I also do my work for free. 😂😂😂 Is it possible to classify different architectures into different support levels? (Maybe keep some architectures of which test results that don't guarantee) If not, I can also try to contact Loongson to afford this.

[shibatch]

I cannot provide ongoing support for free.
If no one is willing to cover the cost, please just fork the project and support LoongArch at your repo.

[wszqkzqk]

I cannot provide ongoing support for free. If no one is willing to cover the cost, please just fork the project and support LoongArch at your repo.

Sigh. As an open source software developer, I completely understand your thoughts. But as a downstream Linux distribution maintainer without financial support, I am also unwilling to bear too much ongoing maintenance costs, so I am unwilling to fork this project. I have already told Loongson about the importance of your project, so let's wait for Loongson's response. 😊😊😊