Enable epoll_pwait2 only when the syscall is available

[michaelrj-google]

Had to be disabled here: #80051

We should enable it on systems that have the syscall, perhaps put this into excludes along with a compile check?

[llvmbot]

@llvm/issue-subscribers-libc

Author: None (michaelrj-google)

Had to be disabled here: https://github.com//pull/80051

We should enable it on systems that have the syscall, perhaps put this into excludes along with a compile check?

[michaelrj-google]

Adding additional context: The epoll_pwait2 syscall was added in linux 5.11 and is unavailable on some of the buildbots. It should stay disabled until all of the buildbots support it.

[nickdesaulniers]

Wont the kernel fail with -ENOSYS if the syscall is unsupported? So I think we should unconditionally provide a decl+def of epoll_pwait2. On pre-5.11 kernels, our wrapper will simply bubble up the ENOSYS from the kernel. On 5.11+, it should work fine.

[nickdesaulniers]

And the unit tests should handle the ENOSYS case.

[michaelrj-google]

The kernel should fail with -ENOSYS of we call a syscall that isn't available, and detecting that and passing the unit tests should be straightforward. The problem is mostly with building, since if the syscall number isn't defined by the headers we currently don't have a way to determine it.