RFR(S): 8220794: PPC64: Fix signal handler for SIGSEGV on branch to illegal address

Hi, Please, could I get reviews for the following change: bug : https://bugs.openjdk.java.net/browse/JDK-8220794 webrev: http://cr.openjdk.java.net/~gromero/8220794/v1/ It fixes the JVM signal handler on Linux / PPC64 when a SIGSEGV generated by a branch to an illegal/invalid address (not mapped address, address with no executable flags, etc) is caught by the JVM signal handler. Currently the signal handler does not handle that case correctly and the JVM crashes silently. That issue was reported by Goetz (SAP). Thanks for reporting the issue, Goetz. Thank you. Best regards, Gustavo