[libc][POSIX] implement poll from poll.h

[nickdesaulniers]

building clang against llvm-libc fails with:

llvm-project-main/llvm/lib/Support/raw_socket_stream.cpp:24:10: fatal error: 'poll.h' file not found.
   24 | #include <poll.h>
      |           ^~~~~~~~

it looks like poll is used.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

building clang against llvm-libc fails with: ``` llvm-project-main/llvm/lib/Support/raw_socket_stream.cpp:24:10: fatal error: 'poll.h' file not found. 24 | #include <poll.h> | ^~~~~~~~ ``` it looks like `poll` is used.

[nickdesaulniers]

libc/test/UnitTest/ExecuteFunctionUnix.cpp also depends on poll, the definition of struct pollfd and POLLHUP, so this is also blocking making the unit tests hermetic. #125092