RFR(XS): 8204652: [aix] fix thread stack allocation, too big stack sizes not recognized

[Lindenmaier, Goetz]

Hi,

This fixes a crash in thread creation with too big stack sizes
http://cr.openjdk.java.net/~goetz/wr18/8204652-aixHugeStacksCrash/01/
Please review.

For Java and Compiler threads the return code of pthread_attr_setstacksize 
 is overwritten. If pthread_attr_setstacksize does not succeed, a tiny 
 stack size with not enough space for the yellow/red pages is allocated. 
 VM crashes with SIGSEGV right away.

Best regards,
  Goetz