RFR [XS] : 8232060: add some initializations using sigemptyset in os_aix.cpp

[Baesken, Matthias]

Hello, please review the following small patch .

Currently a couple of initializations with sigemptyset are missing in os_aix.cpp, that are done in os_linux .
Usually the missing sigemptyset calls should not be a problem, however in special cases (e.g. very seldom failing pthread_* calls later in the coding) they
might lead to uninitialized sigset_t structs used .


Bug/wevrev :

https://bugs.openjdk.java.net/browse/JDK-8232060

http://cr.openjdk.java.net/~mbaesken/webrevs/8232060.0/


Thanks, Matthias