Fix boehmgc-coroutine-sp-fallback.diff for FreeBSD

Our FreeBSD headers have `pthread_getattr_np`, but we get a link-time
error that is missing. The good news is that there is another similar
function which does exist, and the upstream project elsewhere does just
the [fallback code] we need.

As the fallback code indicates, the two functions are not identical
however as the other one needs explicit initialization. NetBSD supports
both in fact, and its [manpage] is therefore a good
resource on what the differences are.

[fallback code]: https://github.com/ivmai/bdwgc/blob/07a6d0ee8889bca5eaeadc13cabadc363725d216/os_dep.c#L1266-L1272

[manpage]: https://man.netbsd.org/pthread_attr_get_np.3

boehmgc-coroutine-sp-fallback.diff

@@ -59,12 +59,18 @@ index b5d71e62..aed7b0bf 100644
      GC_bool found_me = FALSE;
      size_t nthreads = 0;
      int i;
-@@ -851,6 +853,31 @@ GC_INNER void GC_push_all_stacks(void)
+@@ -851,6 +853,37 @@ GC_INNER void GC_push_all_stacks(void)
            hi = p->altstack + p->altstack_size;
            /* FIXME: Need to scan the normal stack too, but how ? */
            /* FIXME: Assume stack grows down */
 +        } else {
-+          if (pthread_getattr_np(p->id, &pattr)) {
++#ifdef HAVE_PTHREAD_ATTR_GET_NP
++          if (!pthread_attr_init(&pattr)
++              || !pthread_attr_get_np(p->id, &pattr))
++#else /* HAVE_PTHREAD_GETATTR_NP */
++          if (pthread_getattr_np(p->id, &pattr))
++#endif
++          {
 +            ABORT("GC_push_all_stacks: pthread_getattr_np failed!");
 +          }
 +          if (pthread_attr_getstacksize(&pattr, &stack_limit)) {