pkg/runtime: ignore runtime.memlimit when allocating heap
pkg/runtime: ignore runtime.memlimit when allocating heap

For Go 1.1, stop checking the rlimit, because it broke now
that mheap is allocated using SysAlloc.  See issue 5049.

R=r
CC=golang-dev
https://codereview.appspot.com/7741050
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go