MIPS: Loongson64: Test register availability before use

Some global register address variable may be missing on
specific CPU type, test them before use them.

Cc: stable@vger.kernel.org
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>