xtensa: add custom bits/poll.h

xtensa: add custom bits/poll.h

Definitions of POLLWRNORM, POLLWRBAND and POLLREMOVE in xtensa linux
kernel are non-standard. Provide bits/poll.h with correct values for
these constants.

This fixes the following strace build errors:

  In file included from xlat/pollflags.h:4:0,
                   from poll.c:34:
  ./static_assert.h:40:24: error: static assertion failed: "POLLWRBAND != 0x0100"
   # define static_assert _Static_assert
                          ^
  xlat/pollflags.h:75:1: note: in expansion of macro ‘static_assert’
   static_assert((POLLWRBAND) == (0x0100), "POLLWRBAND != 0x0100");
   ^~~~~~~~~~~~~
  ./static_assert.h:40:24: error: static assertion failed: "POLLREMOVE != 0x0800"
   # define static_assert _Static_assert
                          ^
  xlat/pollflags.h:117:1: note: in expansion of macro ‘static_assert’
   static_assert((POLLREMOVE) == (0x0800), "POLLREMOVE != 0x0800");
   ^~~~~~~~~~~~~

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>