openrisc: Add thread-local storage (TLS) support

Historically OpenRISC GCC has reserved r10 which we now use to hold
the thread pointer for thread-local storage (TLS).

Signed-off-by: Christian Svensson <blue@cmd.nu>
Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Stafford Horne <shorne@gmail.com>