[libc++abi] Don't set POSITION_INDEPENDENT_CODE when building static library

With the current WebAssembly backend, objects built with -fPIC are not
compatible with static linking.  libc++abi was (mistakenly?) adding
-fPIC to the objects it was including in a static library.

IIUC this change should also mean the static build can be more efficient
on all platforms.

Differential Revision: https://reviews.llvm.org/D60005

llvm-svn: 357322