Make errno a thread-local variable.

errno is supposed to be thread local and we do that here in a rather
makeshift way by implementing a function that returns a pointer to
the __errno variable in the thread block.

If more serious TLS is required, perhaps it would be worthwhile
implementing it using the linker and the "__thread" storage attribute.
That's a bit overkill just for this. I'm also not liking what I saw
the compiler producing.

Change-Id: I03bc0bd6a89f6e3d6bae7653284ee01054614f9a
Reviewed-on: http://gerrit.rockbox.org/803
Reviewed-by: Michael Sevakis <jethead71@rockbox.org>
Tested: Michael Sevakis <jethead71@rockbox.org>

firmware/kernel/include/thread.h

@@ -210,6 +210,7 @@ struct thread_entry
     volatile intptr_t retval;  /* Return value from a blocked operation/
                                   misc. use */
 #endif
+    int __errno;               /* Thread error number (errno tls) */
 #ifdef HAVE_PRIORITY_SCHEDULING
     /* Priority summary of owned objects that support inheritance */
     struct blocker *blocker;   /* Pointer to blocker when this thread is blocked