runtime: new allocation strategy for amd64

runtime: new allocation strategy for amd64

Do not reserve virtual address space.
Instead, assume it will be there when we need it,
and crash loudly if that assumption is violated.
Reserving the address space gets charged to
ulimit -v, which exceeds commonly set limits.

http://groups.google.com/group/golang-dev/msg/7c477af5f5a8dd2c

R=r, niemeyer
CC=golang-dev
https://golang.org/cl/4148045