add PowerPC MemoryBarrier
add PowerPC MemoryBarrier

Fixes issue 3.

R=r
CC=re2-dev
http://codereview.appspot.com/661042

tst2005googlecode/re2,google/re2,nelhage/re2,lnussbaum/re2,Cfretz244/re2,Cfretz244/re2,lamuguo/re2,Distrotech/re2,lnussbaum/re2,bowlofstew/re2,evilmucedin/re2,yanqingmen/re2,AtnNn/re2,lamuguo/re2,wanggchongg/re2,rAntonioh/re2,pombredanne/re2,ElectronicRU/re2,PiRSquared17/re2,lamuguo/re2,tst2005googlecode/re2,driskell/re2-1,bowlofstew/re2,simudream/re2,pombreda/re2,Distrotech/re2,Distrotech/re2,dragonkid/re2,urbit/archaeology2,ElectronicRU/re2,pombreda/re2,zeusdeux/re2,pombredanne/re2,google/re2,zeusdeux/re2,bowlofstew/re2,Cfretz244/re2,urbit/archaeology2,driskell/re2-1,PiRSquared17/re2,wanggchongg/re2,pombredanne/re2,bowlofstew/re2,lnussbaum/re2,google/re2,ElectronicRU/re2,lnussbaum/re2,google/re2,wanggchongg/re2,yanqingmen/re2,pombreda/re2,driskell/re2-1,zeusdeux/re2,nelhage/re2,tst2005googlecode/re2,nelhage/re2,rAntonioh/re2,rAntonioh/re2,AtnNn/re2,urbit/archaeology2,PiRSquared17/re2,simudream/re2,AtnNn/re2,evilmucedin/re2,evilmucedin/re2,google/re2,nelhage/re2,dragonkid/re2,lnussbaum/re2,pombreda/re2,simudream/re2,wanggchongg/re2,driskell/re2-1,lamuguo/re2,dragonkid/re2,evilmucedin/re2,dragonkid/re2,rAntonioh/re2,urbit/archaeology2,yanqingmen/re2,bowlofstew/re2,ElectronicRU/re2,zeusdeux/re2,urbit/archaeology2,pombreda/re2,yanqingmen/re2,lamuguo/re2,PiRSquared17/re2,Distrotech/re2,zeusdeux/re2,urbit/archaeology2,nelhage/re2,driskell/re2-1,wanggchongg/re2,Cfretz244/re2,Cfretz244/re2,rAntonioh/re2,AtnNn/re2,dragonkid/re2,yanqingmen/re2,tst2005googlecode/re2,ElectronicRU/re2,tst2005googlecode/re2,Distrotech/re2,urbit/archaeology2,urbit/archaeology2,pombredanne/re2,PiRSquared17/re2,simudream/re2,nelhage/re2,evilmucedin/re2,pombredanne/re2,AtnNn/re2,urbit/archaeology2,simudream/re2