libc dlfcn.c: make dl_iterate_phdr() from libc more useful

Apparently there are applications that resolve dl_iterate_phdr from libc
and try to call the symbol. Our libc only provides stubs for dl* to
satisfy static linker or statically linked binaries, and is not prepared
to this situation.

Add a code to dso libc to find real dl_iterate_phdr and redirect the
call to it.

Reported by:	yuri
PR:	272992
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

lib/libc/gen/dlfcn.c

@@ -169,7 +169,9 @@ _rtld_thread_init(void *li __unused)
 #ifndef IN_LIBDL
 static pthread_once_t dl_phdr_info_once = PTHREAD_ONCE_INIT;
 static struct dl_phdr_info phdr_info;
+#ifndef PIC
 static mutex_t dl_phdr_info_lock = MUTEX_INITIALIZER;
+#endif
 
 static void
 dl_init_phdr_info(void)
@@ -208,7 +210,16 @@ int
 dl_iterate_phdr(int (*callback)(struct dl_phdr_info *, size_t, void *) __unused,
     void *data __unused)
 {
-#ifndef IN_LIBDL
+#if defined IN_LIBDL
+	return (0);
+#elif defined PIC
+	int (*r)(int (*)(struct dl_phdr_info *, size_t, void *), void *);
+
+	r = dlsym(RTLD_DEFAULT, "dl_iterate_phdr");
+	if (r == NULL)
+		return (0);
+	return (r(callback, data));
+#else
 	tls_index ti;
 	int ret;
 
@@ -223,8 +234,6 @@ dl_iterate_phdr(int (*callback)(struct dl_phdr_info *, size_t, void *) __unused,
 	ret = callback(&phdr_info, sizeof(phdr_info), data);
 	mutex_unlock(&dl_phdr_info_lock);
 	return (ret);
-#else
-	return (0);
 #endif
 }