Allow parallel mark to be enabled on powerpc-linux systems
(Tested using X10 on both BlueGene/Q and Power7/Linux platforms.)

* configure.ac (GC_LINUX_THREADS, PARALLEL_MARK, THREAD_LOCAL_ALLOC):
Define for powerpc*-*-linux* target.
Update AUTHORS file
Check for Fujitsu compiler in builtin_unwind logic (enable FX10/K-Computer)
(Patch originally contributed to X10 project by Kamada-san.)

* include/private/gcconfig.h (HAVE_BUILTIN_UNWIND_INIT): Do not define
if __FUJITSU.
Fix find stackbottom on BlueGene P/Q systems
(Use HEURISTIC2 instead of LINUX_STACKBOTTOM)

* include/private/gcconfig.h (NO_PTHREAD_GETATTR_NP, HEURISTIC2): Define
for BlueGene systems (instead of LINUX_STACKBOTTOM).