[polly][cmake] Don't build LLVMPolly.so without PIC

A build on `sparcv9-sun-solaris2.11` with `-DLLVM_ENABLE_PIC=Off` failed
linking `LLVMPolly.so`:

  [2277/2297] Linking CXX shared module lib/LLVMPolly.so
  FAILED: lib/LLVMPolly.so
  [...]
  ld: fatal: relocation error: R_SPARC_H44: file tools/polly/lib/CMakeFiles/obj.Polly.dir/Analysis/DependenceInfo.cpp.o: symbol .data._ZL16__gthread_active (section): invalid shared object relocation type: ABS44 code model unsupported
  [...]

As on many other targets, one cannot link non-PIC objects into a shared
object on Solaris/sparcv9.

The following patch avoids this by not building the library without PIC.
It allowed the build to finish.

Differential Revision: https://reviews.llvm.org/D85627