Use __builtin_clz() to implement log_2()
Use __builtin_clz() to implement log_2()

A microoptimisation in the block allocator.

vikraman/ghc,vikraman/ghc,mettekou/ghc,shlevy/ghc,snoyberg/ghc,mettekou/ghc,ezyang/ghc,olsner/ghc,olsner/ghc,ezyang/ghc,sdiehl/ghc,mettekou/ghc,olsner/ghc,snoyberg/ghc,sgillespie/ghc,vTurbine/ghc,mettekou/ghc,sgillespie/ghc,snoyberg/ghc,olsner/ghc,vTurbine/ghc,mettekou/ghc,vikraman/ghc,vikraman/ghc,sdiehl/ghc,sgillespie/ghc,vikraman/ghc,shlevy/ghc,snoyberg/ghc,vTurbine/ghc,mettekou/ghc,sgillespie/ghc,ezyang/ghc,sdiehl/ghc,vikraman/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,sgillespie/ghc,shlevy/ghc,sgillespie/ghc,snoyberg/ghc,vTurbine/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,vTurbine/ghc,vTurbine/ghc,olsner/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,olsner/ghc,sdiehl/ghc,vikraman/ghc,olsner/ghc,snoyberg/ghc,vTurbine/ghc,snoyberg/ghc,ezyang/ghc,sgillespie/ghc,mettekou/ghc,ezyang/ghc