LibC: Add some wchar functions

These are basically copy and pasted from the regular string version.
Also add some more multi-byte/wide conversion stub.

libarchive wanted these. There's a lot more, but we can add them
one at a time.

Libraries/LibC/wchar.cpp

@@ -0,0 +1,40 @@
+#include <wchar.h>
+
+extern "C" {
+
+size_t wcslen(const wchar_t* str)
+{
+    size_t len = 0;
+    while (*(str++))
+        ++len;
+    return len;
+}
+
+wchar_t* wcscpy(wchar_t* dest, const wchar_t* src)
+{
+    wchar_t* originalDest = dest;
+    while ((*dest++ = *src++) != '\0')
+        ;
+    return originalDest;
+}
+
+int wcscmp(const wchar_t* s1, const wchar_t* s2)
+{
+    while (*s1 == *s2++)
+        if (*s1++ == 0)
+            return 0;
+    return *(const wchar_t*)s1 - *(const wchar_t*)--s2;
+}
+
+wchar_t* wcschr(const wchar_t* str, int c)
+{
+    wchar_t ch = c;
+    for (;; ++str) {
+        if (*str == ch)
+            return const_cast<wchar_t*>(str);
+        if (!*str)
+            return nullptr;
+    }
+}
+
+}