[libc++abi] Actually set POSITION_INDEPENDENT_CODE when building shared library

This is a bug fix from https://reviews.llvm.org/D60005.

Differential Revision: https://reviews.llvm.org/D60158

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@357550 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed