RFR(XS): 8035423: AIX: Fix os::get_default_process_handle() in os_aix.cpp after 8031968

[Volker Simonis]

Hi,

change https://bugs.openjdk.java.net/browse/JDK-8031968 moved
os::get_default_process_handle() from os_posix.cpp back to the various
platform files but didn't do this for AIX. The fix is trivial - just
insert the implementation of os::get_default_process_handle() into
os_aix.cpp as well:

http://cr.openjdk.java.net/~simonis/webrevs/8035423/
https://bugs.openjdk.java.net/browse/JDK-8035423

This change is currently for jdk9/hs-comp but if 8031968 will be
backported to 8u-dev (which seems to be planned according to the bug
description) we also need this fix in ppc-aix-port/stage (which will
finally go into 8u-dev).

@Vladimir: could you please push this to jdk9/hs-comp/hotspot once it
is reviewed.

Thank you and best regards,
Volker