[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