runtime: correct mmap return value checking on netbsd/openbsd

The current SysAlloc implementation suffers from a signed vs unsigned
comparision bug. Since the error code from mmap is negated, the
unsigned comparision of v < 4096 is always false on error. Fix this
by switching to the darwin/freebsd/linux mmap model and leave the mmap
return value unmodified.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7870044