RFR : 8231753: use more Posix functionality in aix os::print_os_info

[Baesken, Matthias]

Hello, please review the following small (mostly  AIX related)  change .

It replaces the AIX coding  in function
void os::print_os_info(outputStream* st) { ... }


for  uname and load average info  output  by os::Posix functionality.
Additionally it slightly changes  os::Posix::print_load_average  function to handle  the return value of os::loadavg
( indicating  failure ) .

Bug / webrev :

https://bugs.openjdk.java.net/browse/JDK-8231753

http://cr.openjdk.java.net/~mbaesken/webrevs/8231753.0/


Thanks, Matthias