[mlir] Avoid building some shared libraries when PIC is off

When `LLVM_ENABLE_PIC = OFF`, shared libraries cannot be built against code
that's compiled without -fPIC. Example error message:
``
ld.lld: error: relocation R_X86_64_32 cannot be used against local symbol;
recompile with -fPIC
>>> defined in lib/libLLVMSupport.a(StringMap.cpp.o)
>>> referenced by StringMap.cpp
>>>               StringMap.cpp.o:(llvm::StringMapImpl::StringMapImpl(unsigned
>>>               int, unsigned int)) in archive lib/libLLVMSupport.a
``
Similar to [how libclang handles
this](https://github.com/llvm/llvm-project/blob/main/clang/tools/clang-shlib/CMakeLists.txt#L2-L4),
skip building these shared libraries when `LLVM_ENABLE_PIC = OFF`.

Differential Revision: https://reviews.llvm.org/D142941