benchmarks/lmbench: Make os file more robust

It's possible that the previous fix won't work when host version
doesn't match jail version, so recreate the "os" script to ensure
that the PLIST_SUB variable and build directory always match.


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@372108 35697150-7ecd-e111-bb59-0022644237b5