The Linux pattern of adding all the search paths that exist doesn't seem

to suit the FreeBSD folks. Take them back to something closer to the old
behavior. We test whether the /usr/lib32 directory exists (within the
SysRoot), and use it if so, otherwise use /usr/lib.

FreeBSD folks, let me know if this causes any problems, or if you have
further tweaks.

llvm-svn: 148953