RFR : 8218136: minor hotspot adjustments for xlclang++ from xlc16 on AIX

[Baesken, Matthias]

Please review  this small webrev  . It contains a few changes  for  building hotspot   on AIX with  xlclang++  / xlc16  .
( most likely switching to   xlclang++  / xlc16    will be a must once  we introduce C++11/14 features )

Some comments on the changes :


- porting_aix.cpp  :  workaround for demangle.h (does not work with xlclang++)

- arguments.cpp/hpp  :  the UNSUPPORTED_OPTON macro lead  to assigning false to  AllocateHeapAt which is a bad idea (and does not work with xlclang++)

-   globalDefinitions_xlc.hpp   : xlclang++ sets   __GNUC__ so we must not  have #error ... in this case




Bug/webrev :

https://bugs.openjdk.java.net/browse/JDK-8218136

http://cr.openjdk.java.net/~mbaesken/webrevs/8218136.0/


Thanks, Matthias