Use MIUtilSystemLinux on FreeBSD as well

We should later rename this file (probably MIUtilSystemPOSIX), but
more clean-up is still needed here, and we can wait until we better
understand how this code may be shared between FreeBSD, Linux, and OS X.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@209631 91177308-0d34-0410-b5e6-96231b3b80d8