libllvm: add package.

[Redbeanw44602]

Differs from xmake-repo/llvm

1. This package is provided as a library, not a toolchain.
2. Real support for 19.1.7
3. More extensive cross-compilation support.

Packaging principles

Keep llvm-project as default as possible, that is, keep xrepo as little as possible without making choices for the user.

Should splitting packages?

I discussed splitting the package with @star-hengxing in the telegram group, but I later thought that LLVM was more like a whole, so I didn't split it. If there are more opinions, we can discuss it.

[Redbeanw44602]

Possible test failures

I have run the same tests in my own repo and based on my experience, the following test will not pass, but it actually works.

• android (linux, ndk_r22) 4 tests failed. This version of ndk creates huge library files that fill up the disk when building, causing failures. By the way, even if you clean the runner disk space, the compilation cannot be completed. It will eventually be killed due to excessive usage. ndk r22 is really worst!
• freebsd The compilation cannot be completed. In fact, the memory size given by xrepo to the freebsd vm is unreasonable. Ubuntu runner provides 16g of memory while freebsd only has 4g. It will be killed due to oom during tblgen.