runtime: hide mheap from GC It contains pointers to first blocks of lots of spans.
runtime: hide mheap from GC
It contains pointers to first blocks of lots of spans.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/11416046

tv42/old-go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme