This change was adapted from gccgo's libgo/runtime/mem.c at
Ian Taylor's suggestion.  It fixes all.bash failing with
"address space conflict: map() =" on amd64 Linux with kernel
version 2.6.32.8-grsec-2.1.14-modsign-xeon-64.
With this change, SysMap will use MAP_FIXED to allocate its desired
address space, after first calling mincore to check that there is
nothing else mapped there.

R=iant, dave, n13m3y3r, rsc
CC=golang-dev
https://golang.org/cl/4438091