syscall: add mmap test

syscall: add mmap test

This CL adds a test that calls Mmap and Munmap through Syscall9
as the canary that detects assembly fragment breakage. For now
there is no package test that uses Syscall9 in the standard
library across all Unix-like systems.

Note that the package runtime owns its assembly fragments, so
this canary never works for runtime breakage.

LGTM=iant, bradfitz
R=iant, minux.ma, bradfitz
CC=golang-codereviews
https://golang.org/cl/61520049