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/libc/errno.c

@@ -1 +1,5 @@
-int errno;
+#include "thread.h"
+int * __errno(void)
+{
+    return &thread_self_entry()->__errno;
+}

firmware/libc/include/errno.h

@@ -10,10 +10,12 @@
 
 #ifndef _SYS_ERRNO_H_
 
+extern int * __errno(void);
+
 #ifdef PLUGIN
-#define errno   (*rb->__errno)
+#define errno  (*rb->__errno())
 #else
-extern int errno;
+#define errno  (*__errno())
 #endif
 
 #define EPERM 1         /* Not super-user */