java/openjdk8: fix build on powerpc64*

/wrkdirs/usr/ports/java/openjdk8/work/jdk8u-jdk8u302-b08.1/hotspot/src/os_cpu/bsd_ppc/vm/thread_bsd_ppc.cpp:45:37: error: use of undeclared identifier 'uc_mcontext'

frame ret_frame((intptr_t*)uc->>uc_mcontext.mc_gpr[1/*REG_SP*/],
                                ^

/wrkdirs/usr/ports/java/openjdk8/work/jdk8u-jdk8u302-b08.1/hotspot/src/os_cpu/bsd_ppc/vm/thread_bsd_ppc.cpp:88:18: error: redefinition of 'pd_get_top_frame_for_signal_handler'
bool JavaThread::pd_get_top_frame_for_signal_handler(frame* fr_addr, void* ucontext, bool isInJava) {

^

/wrkdirs/usr/ports/java/openjdk8/work/jdk8u-jdk8u302-b08.1/hotspot/src/os_cpu/bsd_ppc/vm/thread_bsd_ppc.cpp:83:18: note: previous definition is here
bool JavaThread::pd_get_top_frame_for_signal_handler(frame* fr_addr, void* ucontext, bool isInJava) {