runtime: implement range access functions in race detector.

Range access functions are already available in TSan library
but were not yet used.

Time for go test -race -short:

Before:
compress/flate 24.244s
exp/norm       >200s
go/printer     78.268s

After:
compress/flate 17.760s
exp/norm        5.537s
go/printer      5.738s

Fixes issue 4250.

R=dvyukov, golang-dev, fullung
CC=golang-dev
https://codereview.appspot.com/7229044