move static-linked stub dlopen out of dynlink.c

src/ldso/dlopen.c

+#include <dlfcn.h>
+#include "libc.h"
+
+__attribute__((__visibility__("hidden")))
+void __dl_seterr(const char *, ...);
+
+static void *stub_dlopen(const char *file, int mode)
+{
+	__dl_seterr("Dynamic loading not supported");
+	return 0;
+}
+
+weak_alias(stub_dlopen, dlopen);

src/ldso/dynlink.c

@@ -1914,11 +1914,6 @@ int dl_iterate_phdr(int(*callback)(struct dl_phdr_info *info, size_t size, void
 	return ret;
 }
 #else
-void *dlopen(const char *file, int mode)
-{
-	error("Dynamic loading not supported");
-	return 0;
-}
 void *__dlsym(void *restrict p, const char *restrict s, void *restrict ra)
 {
 	error("Symbol not found: %s", s);