[libc] Bring libc in-tree

Moving libc from its separate repo into this one, minor resulting build
fixes, and a hacky way to add -I for libc headers in builds.

src/libraries/libc/j6libc/strtox_main.c

@@ -0,0 +1,42 @@
+/* _PDCLIB_strtox_main( const char * *, int, uintmax_t, uintmax_t, int )
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+#include <ctype.h>
+#include <errno.h>
+#include <string.h>
+#include <stdint.h>
+
+uintmax_t _PDCLIB_strtox_main( const char ** p, unsigned int base, uintmax_t error, uintmax_t limval, int limdigit, char * sign )
+{
+    uintmax_t rc = 0;
+    int digit = -1;
+    const char * x;
+    while ( ( x = memchr( _PDCLIB_digits, tolower(**p), base ) ) != NULL )
+    {
+        digit = x - _PDCLIB_digits;
+        if ( ( rc < limval ) || ( ( rc == limval ) && ( digit <= limdigit ) ) )
+        {
+            rc = rc * base + (unsigned)digit;
+            ++(*p);
+        }
+        else
+        {
+            errno = ERANGE;
+            /* TODO: Only if endptr != NULL - but do we really want *another* parameter? */
+            /* TODO: Earlier version was missing tolower() here but was not caught by tests */
+            while ( memchr( _PDCLIB_digits, tolower(**p), base ) != NULL ) ++(*p);
+            /* TODO: This is ugly, but keeps caller from negating the error value */
+            *sign = '+';
+            return error;
+        }
+    }
+    if ( digit == -1 )
+    {
+        *p = NULL;
+        return 0;
+    }
+    return rc;
+}