[libc] Add new libc

This new libc is mostly from scratch, with *printf() functions provided
by Marco Paland and Eyal Rozenberg's tiny printf library, and malloc and
friends provided by dlmalloc.

src/libraries/libc/string/strpbrk.cpp

@@ -0,0 +1,25 @@
+/** \file strpbrk.cpp
+  *
+  * This file is part of the C standard library for the jsix operating
+  * system.
+  *
+  * This Source Code Form is subject to the terms of the Mozilla Public
+  * License, v. 2.0. If a copy of the MPL was not distributed with this
+  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+  */
+
+#include <string.h>
+#include <__j6libc/casts.h>
+
+using namespace __j6libc;
+
+char *strpbrk(const char *s1, const char *s2) {
+    if (!s1 || !s2) return nullptr;
+
+    char const *p = s1;
+    size_t s2len = strlen(s2);
+    while (char c = *p)
+        for (size_t i = 0; i < s2len; ++i)
+            if (c == s2[i]) return cast_to<char*>(p);
+    return nullptr;
+}