RFR(xs): 8144343: [aix] Stack bottom should be page aligned

[Thomas Stüfe]

Hi all,

please review this tiny AIX-only fix. Basically, it takes care of the
thread stack bottom - and hence the placement of the guard pages - to be
always aligned to os::vm_page_size() .

bug: https://bugs.openjdk.java.net/browse/JDK-8144343
webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8144343-aix-stack-bottom-align/webrev.00/webrev/

Kind Regards, Thomas