Fix Android/MIPS build.
Fix Android/MIPS build.

port/atomic_pointer.h was missing an implementation for
MemoryBarrier() for this platform.

Jianbo-Yang/leveldb,zxylvlp/leveldb,zxylvlp/leveldb,olzhabay/PMIndexDB,cmumford/leveldb,luke-jr/leveldb,betterdenger/leveldb,alexxvk/leveldb,matehat/leveldb,norton/leveldb,matehat/leveldb,PFZheng/leveldb,bitcoin-core/leveldb,fastogt/leveldb,WeiHaocheng/LevelDBBuffer,alexxvk/leveldb,jacklicn/leveldb,WeiHaocheng/LevelDBBuffer,bitcoin-core/leveldb,WeiHaocheng/LevelDBBuffer,PFZheng/leveldb,bitcoin/leveldb,olzhabay/PMIndexDB,likema/leveldb,yummyliu/leveldb,bitcoin/leveldb,tianhuhui/leveldb,xuzhezhaozhao/leveldb,likema/leveldb,fastogt/leveldb,fuchsia-mirror/third_party-leveldb,alexxvk/leveldb,Mojang/leveldb-mcpe,norton/leveldb,betterdenger/leveldb,tianhuhui/leveldb,yuwolf/leveldb,jacklicn/leveldb,Mojang/leveldb-mcpe,yuwolf/leveldb,luke-jr/leveldb,PFZheng/leveldb,betterdenger/leveldb,matehat/leveldb,jacklicn/leveldb,Jianbo-Yang/leveldb,dwing4g/leveldb,yummyliu/leveldb,xuerenlv/leveldb,norton/leveldb,xuzhezhaozhao/leveldb,google/leveldb,dwing4g/leveldb,xuerenlv/leveldb,bitcoin/leveldb,Mojang/leveldb-mcpe,bitcoin-core/leveldb,fuchsia-mirror/third_party-leveldb,xuerenlv/leveldb,cmumford/leveldb,dwing4g/leveldb,google/leveldb,tianhuhui/leveldb,olzhabay/PMIndexDB,Jianbo-Yang/leveldb,xuzhezhaozhao/leveldb,yummyliu/leveldb,zxylvlp/leveldb,yuwolf/leveldb