[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/atomax.c

@@ -0,0 +1,25 @@
+/* _PDCLIB_atomax( const char * )
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+#include <string.h>
+#include <ctype.h>
+
+intmax_t _PDCLIB_atomax( const char * s )
+{
+    intmax_t rc = 0;
+    char sign = '+';
+    const char * x;
+    /* TODO: In other than "C" locale, additional patterns may be defined     */
+    while ( isspace( *s ) ) ++s;
+    if ( *s == '+' ) ++s;
+    else if ( *s == '-' ) sign = *(s++);
+    /* TODO: Earlier version was missing tolower() but was not caught by tests */
+    while ( ( x = memchr( _PDCLIB_digits, tolower(*(s++)), 10 ) ) != NULL )
+    {
+        rc = rc * 10 + ( x - _PDCLIB_digits );
+    }
+    return ( sign == '+' ) ? rc : -rc;
+}