[CMake] compiler-rt: Add COMPILER_RT_BUILTINS_ENABLE_PIC

The configuration for -fPIC in the builtins library when built standalone
is unconditional, stating that the flags would "normally be added... by
the llvm cmake step"

This is untrue, as the llvm cmake step checks LLVM_ENABLE_PIC, which allows
a client to turn off -fPIC.

I've added an option when compiler-rt builtins are configured standalone, such
as when built as part of the LLVM runtimes system, to guard the application of
-fPIC for users that want it.

Patch By: JamesNagurne

Differential Revision: https://reviews.llvm.org/D72950