madvise was missing so implement it in assembler. This change
needs to be extended to the other BSD variantes (Net and Open)

Without this change the scavenger will attempt to pass memory back
to the operating system when it has become idle, but the memory is
not returned and for long running Go processes the total memory used
can grow until OOM occurs.

I have only been able to test the code on FreeBSD AMD64. The ARM
platforms needs testing.

R=golang-dev, mikioh.mikioh, dave, jgc, minux.ma
CC=golang-dev
https://golang.org/cl/6850081