runtime: add simple malloc benchmarks

runtime: add simple malloc benchmarks
Allocs of size 16 can bypass atomic set of the allocated bit, while allocs of size 8 can not.
Allocs with and w/o type info hit different paths inside of malloc.
Current results on linux/amd64:
BenchmarkMalloc8	50000000	        43.6 ns/op
BenchmarkMalloc16	50000000	        46.7 ns/op
BenchmarkMallocTypeInfo8	50000000	        61.3 ns/op
BenchmarkMallocTypeInfo16	50000000	        63.5 ns/op

R=golang-dev, remyoudompheng, minux.ma, bradfitz, iant
CC=golang-dev
https://golang.org/cl/9090045