InstrProf: Look for the PIC-version of the profile runtime

If `-shared` is specified, pull in a PIC-version of the profile runtime,
which was added to compiler-rt in r208947.  I'm hoping this will get the
bots on my side.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208948 91177308-0d34-0410-b5e6-96231b3b80d8