RFR(xs): 8171225: [aix] Build gtests on AIX 7.1 with xlC 12

[Thomas Stüfe]

Hi all,

please review this small change. It fixes the gtest build on AIX and
enables it by default.

Note that even though this is a fix for AIX, a cast needed to be added to
shared test coding. This is because xlC struggles with certain template
expansions and I had to help it by providing an explicit cast.

Because linker options were changed as well, this unfortunately this
spreads over two forest parts, so two webrevs were needed.

Issue: https://bugs.openjdk.java.net/browse/JDK-8171225
Webrevs:
(hotspot)
http://cr.openjdk.java.net/~stuefe/webrevs/8171225-aix-build-gtests/webrev.00/webrev/
(top level)
http://cr.openjdk.java.net/~stuefe/webrevs/8171225-aix-build-gtests/toplevel-webrev.00/webrev/

Note that the toplevel change contains the newly generated configure.sh. I
was not sure if that was needed, but it is included for convenience.

Kind Regards, Thomas