[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/filemode.c

@@ -0,0 +1,54 @@
+/* _PDCLIB_filemode( 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 <stddef.h>
+
+/* Helper function that parses the C-style mode string passed to fopen() into
+   the PDCLib flags FREAD, FWRITE, FAPPEND, FRW (read-write) and FBIN (binary
+   mode).
+*/
+unsigned int _PDCLIB_filemode( const char * const mode )
+{
+    unsigned rc = 0;
+    size_t i;
+    switch ( mode[0] )
+    {
+        case 'r':
+            rc |= _PDCLIB_FREAD;
+            break;
+        case 'w':
+            rc |= _PDCLIB_FWRITE;
+            break;
+        case 'a':
+            rc |= _PDCLIB_FAPPEND | _PDCLIB_FWRITE;
+            break;
+        default:
+            /* Other than read, write, or append - invalid */
+            return 0;
+    }
+    for ( i = 1; i < 4; ++i )
+    {
+        switch ( mode[i] )
+        {
+            case '+':
+                if ( rc & _PDCLIB_FRW ) return 0; /* Duplicates are invalid */
+                rc |= _PDCLIB_FRW;
+                break;
+            case 'b':
+                if ( rc & _PDCLIB_FBIN ) return 0; /* Duplicates are invalid */
+                rc |= _PDCLIB_FBIN;
+                break;
+            case '\0':
+                /* End of mode */
+                return rc;
+            default:
+                /* Other than read/write or binary - invalid. */
+                return 0;
+        }
+    }
+    /* Longer than three chars - invalid. */
+    return 0;
+}