[libc-commits] [libc] [libc] document supported os ranges (PR #118863)

[Nick Desaulniers via libc-commits]

nickdesaulniers wrote:

For implications to the codebase, this means that for our syscall wrappers, we have checks that `SYS_<foo>` is defined.  If foo has been supported by all supported architectures for all versions of LTS linux, then we should not clutter the code with prepropecessor guards.

https://www.chromium.org/chromium-os/developer-library/reference/linux-constants/syscalls/ talks a little about this, but doesn't have the kernel version info. I swear on hacker news recently someone linked to a library from chromeos that was trying to do syscall wrappers. It looked interesting, but I cannot find it.

Also, we should stop looking at the host's kernel headers when building llvm-libc for linux, and always require they be passed in. Otherwise, we'll have the wrong syscall numbers when cross compiling.

https://github.com/llvm/llvm-project/pull/118863