fix crash on register stack mark/sweep pass

The crash looks like

  Program received signal SIGSEGV, Segmentation fault.
  mark_locations_array (objspace=0x6000000000045db0, x=0x0, n=864692227966763116) at gc.c:3297
  3297            v = *x;
  (gdb) bt
  #0  mark_locations_array (objspace=0x6000000000045db0, x=0x0, n=864692227966763116) at gc.c:3297
  #1  0x400000000014a040 in gc_mark_locations (objspace=0x6000000000045db0, start=0x0, end=0x6000080000000368) at gc.c:3310
  #2  0x400000000014b3a0 in mark_current_machine_context (objspace=0x6000000000045db0, th=0x60000000000455b0) at gc.c:3500
  #3  0x400000000014dfe0 in gc_mark_roots (objspace=0x6000000000045db0, full_mark=0, categoryp=0x0) at gc.c:4105
  #4  0x400000000014e6b0 in gc_marks_body (objspace=0x6000000000045db0, full_mark=0) at gc.c:4164
  #5  0x400000000014f260 in gc_marks (objspace=0x6000000000045db0, full_mark=0) at gc.c:4526
  #6  0x40000000001525c0 in garbage_collect_body (objspace=0x6000000000045db0, full_mark=0, immediate_sweep=0, reason=256) at gc.c:5024
  #7  0x400000000013c010 in heap_prepare_freepage (objspace=0x6000000000045db0, heap=0x6000000000045dc0) at gc.c:1219
  #8  0x400000000013c140 in heap_get_freeobj_from_next_freepage (objspace=0x6000000000045db0, heap=0x6000000000045dc0) at gc.c:1237
  #9  0x400000000013c360 in heap_get_freeobj (objspace=0x6000000000045db0, heap=0x6000000000045dc0) at gc.c:1259
  #10 0x400000000013c950 in newobj_of (klass=0, flags=40, v1=0, v2=0, v3=0) at gc.c:1303
  #11 0x400000000013ccc0 in rb_newobj_of (klass=0, flags=40) at gc.c:1356
  #12 0x4000000000163740 in hash_alloc (klass=0) at hash.c:289
  #13 0x4000000000163860 in rb_hash_new () at hash.c:309
  #14 0x400000000050e420 in Init_BareVM () at vm.c:2822
  #15 0x40000000000f6b60 in ruby_setup () at eval.c:54
  #16 0x40000000000f6f50 in ruby_init () at eval.c:75
  #17 0x400000000001b010 in main (argc=9, argv=0x60000fffffffb1d8) at main.c:35

The problem here is in call
    gc_mark_locations (objspace=0x6000000000045db0, start=0x0, end=0x6000080000000368) at gc.c:3310
where 'start' (native_main_thread.register_stack_start)
is supposed to be stack start but it's not initialized.

The initialization of 'native_main_thread.register_stack_start'
is supposed to be done in 'ruby_init_stack()'.

But code under 'MAINSTACKADDR_AVAILABLE' exits early.
The fix is to move 'register_stack_start' earlier.

diff --git a/thread_pthread.c b/thread_pthread.c
index c8a7a16..9ad448b 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -722,2 +722,8 @@ ruby_init_stack(volatile VALUE *addr
     native_main_thread.id = pthread_self();
+#ifdef __ia64
+    if (!native_main_thread.register_stack_start ||
+        (VALUE*)bsp < native_main_thread.register_stack_start) {
+        native_main_thread.register_stack_start = (VALUE*)bsp;
+    }
+#endif
 #if MAINSTACKADDR_AVAILABLE
@@ -745,8 +751,2 @@ ruby_init_stack(volatile VALUE *addr
 #endif
-#ifdef __ia64
-    if (!native_main_thread.register_stack_start ||
-        (VALUE*)bsp < native_main_thread.register_stack_start) {
-        native_main_thread.register_stack_start = (VALUE*)bsp;
-    }
-#endif
     {