Revert "[libc++] Do not force building with -fPIC"

This reverts commit r355764.

CMake does not turn -fPIC on for us by default. so this patch breaks
standalone builds. The only reason it hasn't broken any bots is because
LLVM turns on and specifies '-fPIC' for us.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@355756 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed