target/loongarch/cpu: Fix coverity errors about excp_names

target/loongarch/cpu: Fix coverity errors about excp_names

Fix out-of-bounds errors when access excp_names[] array. the valid
boundary size of excp_names should be 0 to ARRAY_SIZE(excp_names)-1.
However, the general code do not consider the max boundary.

Fix coverity CID: 1489758

Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220715060740.1500628-4-yangxiaojuan@loongson.cn>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>