io.c: IO.copy_stream uses poll on Linux

poll and ppoll have a superior API which doesn't require the
kernel to scan a potentially large bitmap to find a high-numbered
FD [ruby-core:35572].  So favor using poll in case IO.copy_stream
encounters a non-blocking FD.

We cannot reliably use poll on most OSes, because file types (e.g.
FIFOs) which work with select may not work with poll.  Fortunately,
Linux uses a common notification mechanism between all
select/poll/epoll variants, so all file types are equally supported
between the notification mechanisms.

Verified by watching strace on the following scripts:

*** maygvl_copy_stream_wait_read ***
require 'io/nonblock'
r, w = IO.pipe
r.nonblock = true
IO.copy_stream(r, "/dev/null")

*** nogvl_copy_stream_wait_write ***
require 'io/nonblock'
r, w = IO.pipe
w.nonblock = true
IO.copy_stream("/dev/zero", w)

* io.c (nogvl_wait_for_single_fd): new function for Linux
  (maygvl_copy_stream_wait_read): Linux-specific version
  (nogvl_copy_stream_wait_write): use nogvl_wait_for_single_fd
  [ruby-core:70051] [Feature #11377]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51305 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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