Add a "fork" of musl as //fusl.

fusl/src/thread/pthread_getattr_np.c

+#define _GNU_SOURCE
+#include "pthread_impl.h"
+#include "libc.h"
+#include <sys/mman.h>
+
+int pthread_getattr_np(pthread_t t, pthread_attr_t *a)
+{
+        *a = (pthread_attr_t){0};
+        a->_a_detach = !!t->detached;
+        if (t->stack) {
+                a->_a_stackaddr = (uintptr_t)t->stack;
+                a->_a_stacksize = t->stack_size - DEFAULT_STACK_SIZE;
+        } else {
+                char *p = (void *)libc.auxv;
+                size_t l = PAGE_SIZE;
+                p += -(uintptr_t)p & PAGE_SIZE-1;
+                a->_a_stackaddr = (uintptr_t)p;
+                while (mremap(p-l-PAGE_SIZE, PAGE_SIZE, 2*PAGE_SIZE, 0)==MAP_FAILED && errno==ENOMEM)
+                        l += PAGE_SIZE;
+                a->_a_stacksize = l - DEFAULT_STACK_SIZE;
+        }
+        return 0;
+}