Have FreeBSD use even more of the same smarts as Linux is now using for

adding search paths. Add them only when they exist, and prefix the paths
with the sysroot. This will allow targeting a FreeBSD sysroot on
a non-FreeBSD host machine, and perhaps more importantly should allow
testing the FreeBSD driver's behavior similarly to the Linux tests with
a fake tree of files in the regression test suite.

I don't have FreeBSD systems handy to build up the list of files that
should be used here, but this is the basic functionality and I'm hoping
Roman or someone from the community can contribute the actual test
cases.

llvm-svn: 148940