[libc++] Do not force building with -fPIC (re-applying)

Summary:
In r355746, we stopped forcing to build with -fPIC because that should
be specified by the CMAKE_POSITION_INDEPENDENT_CODE option at CMake
configure time (and by default -fPIC is used for shared libraries anyways).

However, r355746 had to be reverted in r355756 because we were not
actually building the shared library with -fPIC. The reason is that
we were sharing an object library between the static and the shared
library, which caused flags for static libraries to be used when
building object files that were going to be used for a shared library.

Since this was resolved by r356150, we can stop forcing -fPIC again.

Reviewers: EricWF, smeenai

Subscribers: mgorny, christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D59250

llvm-svn: 356153