[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/strchr.cpp

@@ -0,0 +1,22 @@
+/** \file strchr.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 *strchr(const char *s, int c) {
+    if (!s) return nullptr;
+
+    while(*s && *s != c) s++;
+
+    return *s ? cast_to<char*>(s) : nullptr;
+}