MIPS: rework local_t operation on MIPS64

+. remove "asm/war.h" since R10000_LLSC_WAR became a config option

+. clean up

Suggested-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Huang Pei <huangpei@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>