[CMake] Switch back to -fPIC for Fuchsia Toolchain

This reverts commit 1fab236e.

Shipping LLDB means distributing libLLDB.so, which will need to be built
with PIC. Disabling PIC also removes the libLTO target, which is a hard
dependency of the LLDB test suite.

Reviewed By: abrachet

Differential Revision: https://reviews.llvm.org/D148577