[libc] [llvm] [libc] add remaining epoll functions, pipe (PR #84587)

[Nick Desaulniers via llvm-commits]

================
@@ -0,0 +1,39 @@
+//===---------- Linux implementation of the epoll_create function ---------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/sys/epoll/epoll_create.h"
+
+#include "src/__support/OSUtil/syscall.h" // For internal syscall function.
+#include "src/__support/common.h"
+#include "src/errno/libc_errno.h"
+#include <sys/syscall.h> // For syscall numbers.
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(int, epoll_create, (int size)) {
+#ifdef SYS_epoll_create
+  int ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_epoll_create, size);
+#elif defined(SYS_epoll_create1)
----------------
nickdesaulniers wrote:

Should we be checking for `SYS_epoll_create1` first, then falling back to `SYS_epoll_create`?  I'd imaging that the systems that have `SYS_epoll_create1` also have `SYS_epoll_create`, but not the other way around.  Then again, the man page says these are from Linux 2.6, which is ancient pre-history. We don't need to support systems that old, so perhaps it's worthwhile to do:

```
#if !defined(SYS_epoll_create) || !defined(SYS_epoll_create1)
#error "upgrade your old sh!t"
#endif
int ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_epoll_create1, 0);
```

https://github.com/llvm/llvm-project/pull/84587