IO#advise should not raise Errno::ENOSYS

As it is just a hint the kernel is free to ignore,
IO#advise already succeeds when posix_fadvise is not
available build time at all.  Following that, if posix_fadvise
was available at build time but not implemented in the running
kernel, we should also ignore it.

* io.c (do_io_advise): do not raise on ENOSYS
* test/ruby/test_io.rb (test_advise): do not skip on Errno::ENOSYS
  (test_advise_pipe): ditto
  [ruby-core:72066] [Feature #11806]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53047 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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