runtime: speedup malloc stats collection

runtime: speedup malloc stats collection
Count only number of frees, everything else is derivable
and does not need to be counted on every malloc.
benchmark                    old ns/op    new ns/op    delta
BenchmarkMalloc8                    68           66   -3.07%
BenchmarkMalloc16                   75           70   -6.48%
BenchmarkMallocTypeInfo8           102           97   -4.80%
BenchmarkMallocTypeInfo16          108          105   -2.78%

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/9776043