Try to use /proc on FreeBSD for getExecutablePath

Currently, clang's libTooling passes this function a fake argv0, which
means that no libTooling tools can find the standard headers on FreeBSD.
With this change, these will now work on any FreeBSD systems that have
procfs mounted.  This isn't the right fix for the libTooling issue, but
it does bring the FreeBSD implementation of getExecutablePath closer to
the Linux and macOS implementations.

llvm-svn: 359427