Replace ioctl with select to reduce processor usage by lldb-mi on OSX.

This saga started with a hang on OSX. 2 solutions were proposed.
1) 'select' based solution works ok on OSX but slows down test completion time
on Linux many times.
2) 'ioctl' base solution also works but it causes heavy processor usage on OSX
as reported by Ilia K.

But as the original hang did not occur on Linux so this commit re-introduces the
'select' in conditional code so that it only runs for OSX. There is no need for
this 'fix' to run on Linux.

Initial patch by Ilia K <ki.stfu@gmail.com>. A few changes were made by me. 

llvm-svn: 224258