runtime: fix malloc sampling bug

runtime: fix malloc sampling bug

The malloc sample trigger was not being set in a
new m, so the first allocation in each new m - the
goroutine structure - was being sampled with
probability 1 instead of probability sizeof(G)/rate,
an oversampling of about 5000x for the default
rate of 1 MB.  This bug made pprof graphs show
far more G allocations than there actually were.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5224041