runtime: fix grsec support
runtime: fix grsec support

Changeset 36c9c7810f14 broke support for grsec-patched kernels.
Those do not give back the address requested without MAP_FIXED,
so when verifying an mmap without this flag for success, the
resulting address must not be compared against the requested
address since it may have succeeded at a different location.

R=golang-dev, rsc, gustavo, iant
CC=golang-dev
http://codereview.appspot.com/5650072

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang