This patch broke bootstrap on AIX and probably other targets.

#ifdef ATOMIC_FDE_FAST_PATH
#include "unwind-dw2-btree.h"

static struct btree registered_frames;
static bool in_shutdown;
...
#else

in_shutdown only is defined for ATOMIC_FDE_FAST_PATH but used in code /
asserts not protected by that macro.

  gcc_assert (in_shutdown || ob);
  return (void *) ob;
}

Thanks, David