sw64: fix simd version of memset

Sunway inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5GFOO



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In commit 20b900c4 ("sw64: optimize simd version of memcpy and
memset"), _nc instructions are used to improve performance, but the
position of memb instruction in memset is wrong. Fix it.

Fixes: 20b900c4 ("sw64: optimize simd version of memcpy and memset")
Signed-off-by: Mao Minkai <maominkai@wxiat.com>

Signed-off-by: Gu Zitao <guzitao@wxiat.com>