syscall: add ptrace on darwin

syscall: add ptrace on darwin

The ptrace syscall remains gutted on darwin, but usable.  This
also makes the syscall addition process reproducible on darwin
instead of relying on a file path in rsc's home directory.
I've also removed an override of $PATH in env.bash that made
tooling harder.

R=rsc, dave, r
CC=golang-dev
https://golang.org/cl/4517147