add loong64 support

[cheungxi]

loong64 (loongarch64) a new isa

[yetist]

@gaukas

Please review this patch, otherwise I have to use it like this:

    go mod edit -replace=github.com/refraction-networking/utls=github.com/cheungxi/utls@master
    go mod tidy                                                                  

Thanks.

[gaukas]

Hi @yetist,

I don't really have any major blocker to get this merged. But since loong64 is not even included in go/internal, I'd like to know how are you using other packages on this ISA and what is the specific usage of using uTLS on it.

I guess that's a very personal question, feel free not to respond ;)

And since this PR doesn't really introduce any new function or urgent fix, it might not make a new PR but rather get combined into the next release. Thanks.

And thanks to @cheungxi for contributing!

[gaukas]

And also I don't really have a good way to test on different ISAs, that's a reason I was not merging either this PR or #102. Before I go ahead and merge it, have you checked that this would work instead of breaking anything? Just to make sure.

[VeNoMouS]

Just to clarify... this is to support the cpu arch related to golang/go#46229 ?

[gaukas]

Yes, I believe they are referring to the same architecture by loong64.