Add __popcountsi2 to the function compat list.

This is needed if we use Clang to compile Bionic, which won't include
__popcountsi2 anymore as Clang generates inline instructions. However
prebuilt binary blobs still depend on libc.so to resolve __popcountsi2.

Change-Id: I9001a3884c4be250c0ceebcd79922783fae1a0b7