Tiny alloc memory block is shared by different goroutines running on the same thread.
We call racemalloc after enabling preemption in mallocgc,
as the result another goroutine can act on not yet race-cleared tiny block.
Call racemalloc before enabling preemption.
Fixes #7224.

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/57730043