RFR(s): 8079125: [aix] clean up Linux-specific code remnants in AIX coding

[Thomas Stüfe]

Hi all,

please review this change which cleans up remaining Linux-port remnants
from the AIX port.

Among other things:

- removes a linux-specific workaround for the NPTL-version of
pthread_cond_wait() which found its way to AIX and BSD
- corrects os::readdir()'s usage of readir_r()

Bug: https://bugs.openjdk.java.net/browse/JDK-8079125
webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8079125-aix-cleanup-linux-remnants/webrev.00/webrev/index.html

Thanks, Thomas