Implement -Xss.

        * include/jvm.h (gcj::stack_size): Declare.
        (_Jv_StackSize): Declare.
        * posix-threads.cc (_Jv_InitThreads): Validate gcj::stack_size.
        (_Jv_ThreadStart): Set stack size if specified.
        * prims.cc (gcj::stack_size): Define.
        (parse_memory_size): Renamed from parse_heap_size.
        (_Jv_SetStackSize): Parse stack size argument and set
        gcj::stack_size.

From-SVN: r107132