Update loongarch-base64.xml to match the upstream GDB [1].

[1]:https://github.com/bminor/binutils-gdb/blob/master/gdb/features/loongarch/base64.xml



Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220805033523.1416837-4-gaosong@loongson.cn>