Implement ProcessMonitor::Kill for FreeBSD

On FreeBSD ptrace(PT_KILL) is used to terminate the traced process
(as if PT_CONTINUE had been used with SIGKILL as the signal to be
delivered), and is the desired behaviour for ProcessPOSIX::DoDestroy.

On Linux, after ptrace(PTRACE_KILL) the traced process still exists
and can be interrogated.  It is only upon resume that it exits as though
it received SIGKILL.

For now I'm committing only the FreeBSD change, until the Linux change
(review D3159) is successfully tested.

http://llvm.org/pr18894